Being Human - The Human Accelerated Regions of our genome (HARs).

In the middle of the last decade advances in DNA technology allowed scientists to sequence the genome of the Chimpanzee (1). The door was thus open to compare the human genome and that of our nearest relative. The aim was to identify what changes have led to humanity's unique abilities. Of particular interest were our cognitive abilities, speech and language and upright mode of locomotion.

A major step towards this goal was achieved when Pollard publish a pair of papers in 2006 (2 and 3).

In the first paper Pollard (2) and her co-authors said the following:

 

“Recent sequencing and assembly of the genome of the common chimp (Pan troglodytes) offers an unprecedented opportunity to understand primate evolution and to identify those changes in the ancestral hominoid genome which gave rise to the modern human species [1]. Primate genome comparisons are expected to shed light on questions as diverse as the origins of speech [2,3] and the progression of HIV infection to AIDS [4]. Whereas the aim of comparative studies of human and rodent genomes [5,6] is typically to identify genomic elements that are evolutionarily conserved (and therefore presumably functionally important given the ~150 million years of evolution separating the species), we look to the chimpanzee genome to better understand what is uniquely human about our genome. One goal is to find DNA elements that show evidence of rapid evolution in the human lineage, where “accelerated” or “rapid” refers to a general increase in the rate of nucleotide substitution. Pollard et al. [7] used comparative genomics to identify 49 such human accelerated regions (HARs) that are evolving very slowly in vertebrates but have changed significantly in the human lineage. The most accelerated of these, HAR1, was found to be a novel RNA gene expressed during neocortical development [7]. In this paper, we investigate the properties of a larger set of 202 carefully screened HARs in order to unravel the evolutionary forces at work behind the fastest evolving regions of the human genome.

 

To address questions of human-specific molecular evolution it is not sufficient to simply identify all nucleotide differences between the human and chimpanzee genomes. Despite being a small fraction of the human genome, the number of human bases that differ from the corresponding chimp base is still large (nearly 29 million bases), and it is likely that most of these differences do not have a functional consequence. Furthermore, many authors, starting with the seminal work of King and Wilson [8], have suggested that the majority of the changes that distinguish humans from other hominoids will be found in the 98.5% of the genome that is non-coding DNA, which is a vast territory to search. To identify changes that may be functional, we focus on the set of regions of the human genome of at least 100 base pairs (bp) that appear to have been under strong negative selection up to the common ancestor of human and chimp (as evidenced by high sequence identity between chimp and rodents), but exhibit a cluster of changes in human compared to chimp. Our expectation is that the selective constraint on the most extremely accelerated regions of the human genome may have switched from negative to positive (and possibly back to negative) some time in the last 5−6 million years.”

Note To allow the reader to fully appreciate Pollard et. al’s argument I have included the ‘nested’ references from the above under a sub-heading “Pollard 2 references” in my references section at the end of this post, my own reference appear in round (n) brackets.

 

Put simply what Pollard et. al. were saying was:

·         In placental mammals certain genomic areas have been conserved over a vast stretch of time - 150 million years.

·         The most likely reason for long period of conservation is that they are “functionally important”.

Yong (9) neatly describes functionally important DNA thus: “For years, we’ve known that only 1.5 percent of the genome actually contains instructions for making proteins, the molecular workhorses of our cells. But ENCODE has shown that the rest of the genome – the non-coding majority – is still rife with “functional elements”. That is, it’s doing something.

It contains docking sites where proteins can stick and switch genes on or off. Or it is read and ‘transcribed’ into molecules of RNA. Or it controls whether nearby genes are transcribed (promoters; more than 70,000 of these). Or it influences the activity of other genes, sometimes across great distances (enhancers; more than 400,000 of these). Or it affects how DNA is folded and packaged. Something.”

·         The authors compared the conserved areas of the Chimp and human DNA to look for areas of rapid evolution these they named “human accelerated regions” (HARs).

·         They found 202 such HAR regions of DNA.

·         To identify functional changes in the human genome as compared to that of the Chimp the authors focus on these HARs.

 

The paper’s results show the following

 

·         The normalized human substitution rate exceeds the rate in the chimp-rodent phylogeny in all of the HARs.

·         The divergence between the human and chimpanzee genomes is higher in the top 49 HARs

·         Directly comparing substitution rates per site in the human and chimp branches (over the same period of evolutionary time), the human rate is an average of seven times higher than the chimp rate in HAR1–HAR5.

·         The HAR elements themselves are significantly more diverged from chimpanzee than surrounding sequences

·         The index of dispersion (i.e., the ratio of the variance in the number of substitutions on a lineage to the mean number).. in HAR1–HAR5 is much larger than the expected value of 1.. and therefore these data are compatible with strong selection on the human lineage.

·         All of the observed human-specific changes in HAR1–HAR5 occurred after human diverged from chimp.

·         These findings are in agreement with the hypothesis, first proposed by King and Wilson in 1975, that the majority of chimp-human phenotypic differences can be explained by differential control of transcriptional networks [8] which may be expected to occur primarily in the non-coding DNA and in particular in the HAR regions identified (own italics).

In her second paper of 2006 Pollard et. al. looked more closely at the top ranked region of significant evolutionary acceleration. They reported that the most dramatic of these ‘human accelerated regions’, HAR1 "is part of a novel RNA gene (HAR1F) that is expressed specifically in Cajal–Retzius neurons in the developing human neocortex from 7 to 19 gestational weeks, a crucial period for cortical neuron specification and migration. HAR1F is co-expressed with reelin, a product of Cajal–Retzius neurons that is of fundamental importance in specifying the six-layer structure of the human cortex".

In other words the change in the HAR1 region is more than like responsible for humanity's differences with respect to higher functions such as sensory perception, generation of motor commands, spatial reasoning, conscious thought, and language.

The impact of these two papers was immense. The identification of the HAR regions opened the door for researchers to investigate the differences between our nearest hominid relative the Chimpanzee and answer the question what TRULY makes us human.

This is all well and good, but HOW did Pollard et. al. accomplish this? Her popular science piece of 2012 for Scientific American (4) explains the process of hunting for the differences between the Chimp and Human genomes:

“To facilitate the hunt, I wrote a computer program that would scan the human genome for the pieces of DNA that have changed the most since humans and chimps split from a common ancestor. Because most random genetic mutations neither benefit nor harm an organism, they accumulate at a steady rate that reflects the amount of time that has passed since two living species had a common forebear (this rate of change is often spoken of as the “ticking of the molecular clock”). Acceleration in that rate of change in some part of the genome, in contrast, is a hallmark of positive selection, in which mutations that help an organism survive and reproduce are more likely to be passed on to future generations. In other words, those parts of the code that have undergone the most modification since the chimp-human split are the sequences that most likely shaped humankind.

In November 2004, after months of debugging and optimizing my program to run on a massive computer cluster at the University of California, Santa Cruz, I finally ended up with a file that contained a ranked list of these rapidly evolving sequences.”

 

Pollard further explains what she did next:

 

“We spent the next year finding out all we could about the evolutionary history of HAR1 by comparing this region of the genome in various species, including 12 more vertebrates that were sequenced during that time. It turns out that until humans came along, HAR1 evolved extremely slowly. In chickens and chimps—whose lineages diverged some 300 million years ago—only two of the 118 bases differ, compared with 18 differences between humans and chimps, whose lineages diverged far more recently. The fact that HAR1 was essentially frozen in time through hundreds of millions of years indicates that it does something very important; that it then underwent abrupt revision in humans suggests that this function was significantly modified in our lineage.”

 

The result was the two papers outline above. A nice illustration accompanies her earlier 2009 piece (5) also in scientific American.

 

Photo credit: Pollard (5)

 

Since then a huge amount of research has gone into looking at the HARs. In 2012 Pollard, herself  (4) summarised these:

 

“HAR1 resides in two overlapping genes. The shared HAR1 sequence gives rise to an entirely new type of RNA structure, adding to the six known classes of RNA genes. These six major groups encompass more than 1,000 different families of RNA genes, each one distinguished by the structure and function of the encoded RNA in the cell... HAR1 is also the first documented example of an RNA-encoding sequence that appears to have undergone positive selection..”

 

“So, too, is the FOXP2 gene, which contains another of the fast-changing sequences I identified and is known to be involved in speech. ..FOXP2 extracted from a Neandertal fossil and found that these extinct humans had the modern human version of the gene, perhaps permitting them to enunciate as we do.”

 

“.. human brain volume has more than tripled since the chimp-human ancestor—a growth spurt that genetics researchers have only begun to unravel.
One of the best-studied examples of a gene linked to brain size in humans and other animals is ASPM. Genetic studies of people with a condition known as microcephaly, in which the brain is reduced by up to 70 percent, uncovered the role of ASPM and another gene—CDK5RAP2—in controlling brain size. More recently, researchers at the University of Chicago, the University of Michigan and the University of Cambridge have shown that ASPM experienced several bursts of change over the course of primate evolution, a pattern indicative of positive selection. At least one of these bursts occurred in the human lineage since it diverged from that of chimps and thus was potentially instrumental in the evolution of our large brains.. Amazingly, more than half of the genes located near HARs are involved in brain development and function..”

 

A little more detail on HAR1 activity from Carta Anthropology (6):

 

“Human Accelerated Regions 1 (HAR1) is part of the cis-antisense RNA gene pair HAR1F and HAR1R, which are expressed in neurons during human embryonic cortical development and adult brain. HAR1 is conserved in amniotes as far back as frogs, but 18 base pair substitutions have occurred specifically in the human lineage leading to a secondary structure change in HAR1F that is unique to humans. HAR1F co-expresses with reelin, a protein important to the proper layering of the human cortex, suggesting an important role for HAR1 in human brain development. In addition, HAR1 expression is repressed by REST, and it has been hypothesized that changes in HAR1 expression may contribute to Huntington’s disease phenotypes.”

Photo credit: Pollard (5)

Back to Pollard’s 2012 article. Having pointed out some of the positive effects of the Human Accelerated Regions of our genome, Pollard notes that there are some negative consequences associated with HARs:

 
PtERV1 is a relic retro-virus that plagued ancient chimps, gorillas and humans living in Africa about four million years ago. Its effects can be found on the genes we have inherited from our ancestors.

Researchers reconstructed the original PtERV1 sequence and re-created this ancient retrovirus. They then performed experiments to see how well the human and great ape versions of the TRIM5α gene could restrict the activity of the resurrected PtERV1 virus. Their results indicate that most likely a single change in human TRIM5α enabled our ancestors to fight PtERV1 infection more effectively than our primate cousins could, however these same shifts make it much harder for us to fight HIV. This finding is helping researchers to understand why HIV infection leads to AIDS in humans but less frequently does so in nonhuman primates.

 

Photo credit: Pollard (5)

In 2014 Pollard also co-authored a review article (7) with Hubisz on the work carried out on HARs

 

“Transgenic [gene regulatory] enhancer assays also enable the activity of a human ncHAR sequence to be compared to its ortholog from chimpanzee or other mammals. Of 26 ncHAR enhancers that have been tested using both human and non-human primate sequences, seven drive human-specific expression patterns in mouse embryos at day 11.5. The tissues with differential expression are limb (HAR2, 2xHAR114), eye (HAR25), forebrain (2xHAR142, 2xHAR238), and the midbrain–hindbrain boundary (2xHAR164, 2xHAR170). The functional implications of these expression differences remain to be discovered, but it is tempting to speculate that changes in the development of these tissues could influence human anatomy and traits such as fine motor skills, spoken language, and cognition.”

 
In other words non-coding (nc)HARs are implicated in limb, eye, fore, mid and hindbrain development and thus may affect the development of fine motor skills, language and the higher reasoning skills seen in humanity.

The 2012 article in Scientific American had some people bamboozled though. One online comment (8) in particular, made me smile:

“If the 118 base pair sequence that makes up HAR1 have been so highly conserved over 300 million years with only 2 base pair substitutions since chickens and chimps diverged, what type of natural selection process could account for 18 base pair changes in the span of 6 millions years since we split with the chimps. And why haven't we seen examples of 4, 6, 8 or more base pair variations in any other species? Is it possible that the only viable genetic variation for the HAR1 sequence would be the ancestral and the human versions, with nothing in between? If so, what are the odds that random mutation could be responsible for 18 base pair changes all occurring at the same time in such a highly conserved piece of DNA code? I think these questions should be answered by the author!”

Although at the time, a little logic was needed the commenter could have answered his own question with a little thought.. During the estimated six million (too low a number in my opinion) years since our split from our last common ancestor with Chimpanzees, humanity has been through/on a HUGE genetic odyssey. What species led to and/or contributed genes to, humanity among Ardipithecus, Australopithecus, Homo habilis, Homo erectus, Homo ergastor, Homo heidelburgensis and lastly Homo neaderthalensis is still an open question. However since the interbreeding between Neanderthals and humans has been discovered through the sequencing of Neanderthal genomes, much work has been carried out to understand in what species HARs first began to appear. In their review article (7) Hubisz and Pollard give their take on when HARs first appear on the human tree:

 

“Genomes from archaic hominins and diverse modern humans provide information about when along the human lineage HAR mutations arose. We analyzed ncHARs for mutations shared with a Neanderthal [11] and a Denisovan [12] using other primates (100-way alignments;http://genome.ucsc.edu) to polarize differences. We estimate that 7.1% of human–chimp differences in ncHARs occurred after divergence from archaic hominins and 2.7% are shared. The post-archaic fraction is similar to that observed in targeted sequencing of HARs captured from an Iberian Neanderthal fossil [31^•]. Compared to chimp–human differences in flanking regions and phastCons elements, those in ncHARs are significantly more likely to be pre-archaic (90% show derived allele only in Neanderthal and Denisovan; both P < 0.01). Thus, the archaic hominins provide some evidence for a depletion of accelerated evolution in the past ∼1 million years of human evolution compared to earlier in our lineage.”

(note: nested references in this passage are given below under ‘Hubisz and Pollard references’ below)

Basically my reading of the above is that 92.9% (100-7.1) of HARs occurred BEFORE the split between the ancestors of modern humans and archaic species. In other words MOST of our evolution had occurred way BEFORE we, modern humans, emerged in Africa ca. 200000 years ago.. Whoa! Now there’s something to ponder!

 

So to answer the commenter query: “Of course the number of base pair changes didn’t happen all at once, they happened during our genetic journey.. but mainly early on”.

 

Readers of my blog may wonder where I am going with all this in the light of the other types of post on this blog.. Well all I can say for now is read the statement of intent at the top of the blog and give it a guess. Best answer wins a copy of ‘The Last Giant of Beringia’..
 

References 

1. The Chimpanzee Sequencing and Analysis Consortium.
Chimpanzee Sequencing and Analysis Consortium (2005) Initial sequence of the chimpanzee genome and comparison with the human genome. Nature 437: 69–87.
Full article available at
http://www.nature.com/nature/journal/v437/n7055/full/nature04072.html
 
2. Pollard K.S., Salama S.R., King B., Kern A.D., Dreszer T., Katzman S., Siepel A., Pedersen J.S., Bejerano R., Baertsch R., et al.
Forces shaping the fastest evolving regions in the human genome. PLoS Genet. 2006;2:1599–1611.
Found at http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.0020168#pgen-0020168-b007
 
3. Pollard K.S., Salama S.R., Lambert N., Lambot M.A., Coppens S., Pedersen J.S., Katzman S., King B., Onodera C., Siepel A., et al. An RNA gene expressed during cortical development evolved rapidly in humans. Nature. 2006a;443:167–172
Abstract available at http://www.ncbi.nlm.nih.gov/pubmed/16915236
 
4. Pollard K.S., 2012. Secrets of Our Success. What makes us different? Scientific American Volume 22, Issue 1s
 
5. Pollard K.S. What makes us Human? Sci Am. 2009 May; 300(5):44-9.
 
6. Carta Anthropology.  Retrieved from:
http://carta.anthropogeny.org/moca/topics/human-accelerated-region-1-har1
 
7. Melissa J Hubisz and Katherine S Pollard. Exploring the genesis and functions of Human Accelerated Regions sheds light on their role in human evolution. Current Opinion in Genetics & Development 2014, 29:15–21
Download at http://www.sciencedirect.com/science/article/pii/S0959437X14000781
 
8. Comments on “What makes us Human”. Retrieved from:
http://www.scientificamerican.com/article/what-makes-us-human/
 
9. ENCODE: the rough guide to the human genome By Ed Yong 9/5/2012
Found at http://blogs.discovermagazine.com/notrocketscience/2008/06/14/rna-gene-separates-human-brains-from-chimpanzees/

 

Pollard 2 References
1. Chimpanzee Sequencing and Analysis Consortium (2005) Initial sequence
of the chimpanzee genome and comparison with the human genome.
Nature 437: 69–87.

2. Enard W, Przeworski M, Fisher S, Lai C, Wiebe V, et al. (2002) Molecular

evolution of FOXP2, a gene involved in speech and language. Nature 418:
869–872.

3. Holden C (2004) The origin of speech. Science 303: 1316–1319.

4. Varki A (2000) A chimpanzee genome project is a biomedical imperative.

Genome Res 10: 1065–1070.

5. Waterston R, Lindblad-Toh K, Birney E, Rogers J, Abril JF, et al. (2002)

Initial sequencing and comparative analysis of the mouse genome. Nature
420: 520–562.

6. Rat Genome Sequencing Project (2004) Genome sequence of the brown

Norway rat yields insights into mammalian evolution. Nature 428: 493-521.

7. Pollard KS, Salama SR, Lambert N, Coppens S, Pedersen JS, et al. (2006) An

RNA gene expressed during cortical development evolved rapidly in humans. Nature.
E-pub ahead of print 16 August 2006.

8. King MC, Wilson AC (1975) Evolution at two levels in humans and

chimpanzees. Science 188: 107–116.

Hubisz and Pollard references11. K. Prufer, F. Racimo, N. Patterson, F. Jay, S. Sankararaman, S. Sawyer, A. Heinze, G. Renaud, P.H. Sudmant, C. de Filippo, et al. The complete genome sequence of a Neanderthal from the Altai Mountains. Nature, 505 (2014), pp. 43–49
Pdf download available at http://dash.harvard.edu/handle/1/12717373?frbrVersion=10
 
12. M. Meyer, M. Kircher, M.T. Gansauge, H. Li, F. Racimo, S. Mallick, J.G. Schraiber, F. Jay, K. Prufer, C. de Filippo, et al. A high-coverage genome sequence from an archaic Denisovan individual. Science, 338 (2012), pp. 222–226

31. H.A. Burbano, R.E. Green, T. Maricic, C. Lalueza-Fox, M. de la Rasilla, A. Rosas, J. Kelso, K.S. Pollard, M. Lachmann, S. Paabo
Analysis of human accelerated DNA regions using archaic hominin genomes
PLoS ONE, 7 (2012), p. e32877
Full Text via CrossRef
 

The Peopling of the Americas II: Bering Land Bridges of the Pleistocene

Highlights

 
·         Number of Bering Land Bridges and dates shown graphically and tabulated for the majority of the Pleistocene

Example

In my last post (see here) I detailed the research leading up to the confirmation of the formation the Bering Land Bridges. I also explored the literature on the Paleoenvironment extant on the various Bering Land Bridges formed during the Pleistocene. The main points were:

·         The Bering Land Bridge is believed to have formed between 9 and 20 times during the Pleistocene (Hopkins¹ and Pielou² respectively)

·         That 4 waves of mammalian immigration had occurred over the land bridge, once during early Miocene, again at the Pliocene-Pleistocene boundary and probably at least twice more during Pleistocene. Furthermore the climate on the land bridge appeared to be temperate, humid and forested in the first two episodes, temperate grasslands in the mid Pleistocene and only in late Pleistocene was the climate significantly colder being represented by steppe tundra, ‘alpine desert’ plant communities. Additionally there was some back migration from the Americas to Asia.

The main question remaining is therefore: At what dates during the Pleistocene was the land bridge above sea level and therefore available for the migration of animals and most importantly humans into the Americas?

 

Not unexpectedly much research has attempted to address this question. Whilst the first opening of the Bering seaway in the Miocene, and its final reopening at the end last glacial maximum (LGM) has been relatively well dated by, for example, but not limited to, Marincovich and Gladenkov³ and Elias et. al.⁴ respectively.

Direct evidence of past sea levels and hence the emergence and submergence of the Bering Land Bridge is however, rather scant and indirect.

For this post I reviewed a range of evidence regarding sea level across the Pleistocene including papers by Brigham-Grette et. al.⁵, Roeda, et. al.⁶, Goodfriend et. al.⁷ and Scherer et. al.⁸ amongst many others. None of the papers I could find and/or access fully gave enough evidence to infer a chronology for Bering Land Bridge formation.

Another line of evidence to infer land bridge existence at a particular epoch is the mammalian fauna migrations. If anything these are even more fragmentary than the sea level evidence. As an example, in a review of the work of Reppening on arvicoline, rodents migrations and species, Bell and Jass⁹ had 68 references spread across 22 authors! And that’s just a small section of the rodents! To provide a synthesis of the evidence of mammalian migrations to the Americas would therefore, be a monumental undertaking, and beyond the scope of this post.

General references, however state the rough timings of two important species migrations to the Americas, in “When Did Columbian Mammoths Come to North America?”¹⁰ The author states:

“The ancestors of Columbian mammoths lived in Asia and came to North America about 1.8 million years ago across the Bering land bridge (see the map below). This land bridge was between Russia and Alaska. The Columbian mammoth moved throughout the United States and parts of Mexico. They never went south of Mexico.

The woolly mammoth also came to North America from Asia across the Bering land bridge. They started coming to North America 100,000 years ago and stayed in the north, remaining in Alaska and Canada.”

 

 

Image credit: Children’s Discovery Museum of San Jose¹⁰

Original caption reads: Migration patterns of Columbian and Woolly Mammoths.

Noteworthy is the agreement of these dates with those of Reppening¹¹ of nearly 50 years ago.

 

Some recent research¹² however, indicates early dates of entry for other proboscids  museum “staff were working at the late Miocene-age (7.0-4.5 million years old) locality just 20 minutes from the university in eastern-most Tennessee and came across some tusk fragments!  ..the long, straight tusk – it will be over 2.5 m (8 ft) long when complete. It is not highly curved such as the mammoth tusks we have here in The Mammoth Site.  Following the tusks back into the excavation wall, staff members Shawn Haugrud and Brian Compton located the skull and lower jaws. Further excavation allowed them to discover the articulated neck vertebrae…and it keeps going.  There is more of the animal but under lots of in-situ sediments yet to be excavated! Look at the teeth.  Note the cusps, ridges, and valleys – these are buno-lophodont teeth.  These are not like the flat-grinding teeth of our mammoths or today’s elephants.  We are not sure yet of a detailed identification but we do know that these teeth are from a mastodont” [Mastodon early relative].

Image credit: Mammoth Site of Hot Springs South Dakota¹²
Original caption reads: Here is a close-up, side view of the upper cheek teeth. Note the cusps, lophs, and valleys on these mastodont teeth.  These teeth do not grind grasses and sedges as do mammoths but chomp up woody plants while browsing in woodlands and forests.

Image credit: Mammoth Site of Hot Springs South Dakota¹²

Original caption reads: One of four adult tusk fragments so far recovered that belong to a large mastodont from the late Miocene at the Gray Fossil Site, eastern Tennessee.

Lastly the final mammalian migrations - including that of humans - occurred, according to a recent paper¹³ up until the last glacial cycle c. 11000 year ago.

Image credit: Meiri et. al. (2014)¹³

Original caption reads: Figure 1. (a) Bayesian phase-modelled timing of the late-glacial colonization of Alaska and Yukon by brown bears, cave lions, moose, wapiti and humans. The distributions are start boundaries. (b) Finite radiocarbon dates of wapiti occupying northeast Siberia plotted against NorthGRIP ẟ ¹⁸O data.

 

Other lines of investigation, particularly ocean sediment cores could give direct evidence of these events. As a relatively recent report¹⁴ states of the Bering strait region:

“This is the only area on Earth where the circulation between ocean basins has been blocked and a migration corridor between continental landmasses has been opened by falling sea levels of the Pliocene and Pleistocene epochs, yet scientific drilling for the purpose of paleoclimate analysis has never been conducted in the Bering Strait region. ..In order to address unresolved questions regarding global ocean circulation and rapid climate changes, and to permit reconstruction of the flora, fauna, and climate of the Bering Land Bridge, basinal features that contain both marine and terrestrial lacustrine sediment must be targeted. ..Norton and Hope basins are most proximal to the Bering Strait, constituent records of Pleistocene and Holocene transgressions and regressions from any of the nine basins (Fig. 1) would serve to constrain temporal estimates of the opening and closing of the Bering Strait..”

 

Despite all these decades of work, by a myriad of scientists, using multiple lines of evidence, direct dates for the timings of emergence and submergence of the Bering Land Bridges STILL haven’t been arrived at!

 

There is however ONE method that can estimate sea levels globally and hence allow us to derive dates for the emergence and submersion of the Bering Land Bridge.

 

The method is based the Marine Isotope Stage data. A good description from Wikipedia:

“Marine isotope stages (MIS), marine oxygen-isotope stages, or oxygen isotope stages (OIS), are alternating warm and cool periods in the Earth's paleoclimate, deduced from oxygen isotope data reflecting changes in temperature derived from data from deep sea core samples. Working backwards from the present, which is MIS 1 in the scale, stages with even numbers have high levels of oxygen-18 and represent cold glacial periods, while the odd-numbered stages are troughs in the oxygen-18 figures, representing warm interglacial intervals. The data are derived from pollen and foraminifera (plankton) remains in drilled marine sediment cores, sapropels, and other data that reflect historic climate; these are called proxies.

The MIS timescale was developed from the pioneering work of Cesare Emiliani in the 1950s, and is now widely used in archaeology and other fields to express dating in the Quaternary period (the last 2.6 million years), as well as providing the fullest and best data for that period for paleoclimatology or the study of the early climate of the earth, representing the standard to which we correlate other Quaternary climate records. Emiliani's work in turn depended on Harold Urey's prediction in a paper of 1947 that the ratio between oxygen-18 and oxygen-16 isotopes in calcite, the main chemical component of the shells and other hard parts of a wide range of marine organisms, should vary depending on the prevailing water temperature in which the calcite was formed.”

 

This data can be used to estimate sea levels as explained by Lambek et. al¹⁶: “The isotope ratio ẟ¹⁸O={(¹⁸O/¹⁶O)_sample/(¹⁸O/¹⁶O)_standard, expressed as parts per thousand, is therefore believed to be an indicator of global ice volume — low values indicate small ice volumes and hence globally warm conditions, and high values imply large ice sheets and low temperatures..” [and by extension global sea levels].

 

Climate scientists have therefore combined many of oxygen isotope ratio data sets from deep sea cores to obtain values for global ice volume and thus infer eustatic global sea levels. Some authors have produced graphs of sea level verses time for considerable past epochs. Three of the best these are Siddal¹⁷, Compton¹⁸ and Lambeck et. al.¹⁶.

 

If we combine this with Hopkins assertion that ”Sea level would have to fall only 46 meters below its present position to expose a narrow land connection between Chukota and Alaska by way of St. Lawrence Island; a reduction to -50 metres would expose a second narrow connection north of the Bering Strait..” (ref. 1 p460).

 

We can superimpose a -50m below present, line on their graphs and thus find the dates and numbers of Bering Land Bridges during the latter (1.8Mya to present) when modern humans or previous species of Homo such as Homo neanderthalis or Homo erectus s.l. could conceivably have crossed into the Americas. See graphs below.

Fig 1. Bering Land Bridges Present to 800Ky BP. Adapted from Siddal¹⁷

 

Fig 2. Bering Land Bridges 800-1800Ky BP. Adapted from Compton¹⁸

Fig 3. Bering Land Bridges Present to 150Ky BP. Adapted from Lambeck¹⁶

I have summarised the data in table form for ease of viewing/use enjoy!

 

 

 

 

There are some profound implications of these data, which I will expand on in a future post.

 

Note Bering Land Bridges 1-6 were generated using the more detailed Fig 3 to show finer scaling of Land Bridge emergences whilst Land Bridges 7-39 were drawn from Figs. 1 and 2, consequently Land Bridges 7 and 8 show some overlap. This is not an error, just a result of the finer scaling of Fig 3 vs Fig 1.
 

References

1. Hopkins, D.M. 1967. The Cenozoic history of Beringia—A Synthesis., in The Bering Land Bridge Hopkins, D.M, ed. Stanford University Press.

 

2. Pielou, E.C. 1991.  After the Ice Age: The Return of Life to Glaciated North America. University of Chicago Press, Chicago

 

3. Louie Marincovich, Jr & Andrey Yu. Gladenkov. 1999. Evidence for an early opening of the Bering Strait. Nature 397, 149-151

 

4. Elias, Scott A. et al. 1996. Life and times of the Bering land bridge

Nature 382, 60 - 63 doi:10.1038/382060a0

 

5. Brigham-Grette, J., and Hopkins, D.M., Benson, S.L., Heiser, P., Ivanov, V.F., Basilyan, A., and Pushkar, V., 1995, Coastal records of Pleistocene Glacial and Sea level events on Chukotka Peninsula, northeast Siberia: A new interpretation, Current Research in the Pleistocene --Special issue on Beringia, v.11.

 

6. Roeda, Murray A. et. al. 2013. Evidence for an Early Pleistocene glaciation in the Okanagan Valley, southern British Columbia. Canadian Journal of Earth Sciences v. 51  no. 2  p. 125-141

 

7. Goodfriend, G. A., J. Brigham-Grette, and G. H. Miller, 1996, Enhanced age resolution of the Marine Quaternary Record in the Arctic using Aspartic Acid Racemization dating of Bivalve shells, Quaterary Research,v. 45, 176-187.

 

8. Scherer, Reed P. et. al. 1998. Pleistocene Collapse of the West Antarctic Ice Sheet. Science Vol. 281 no. 5373  pp. 82-85. DOI: 10.1126/science.281.5373.82

 

9. Bell, Christopher J., and Jass, Christopher N. 2011. Polyphyly, paraphyly, provinciality, and the promise of intercontinental correlation: Charles Repenning’s contributions to the study of arvicoline rodent evolution and biochronology. Palaeontologia Electronica Vol. 14, Issue 3; 18A:15p;

palaeo-electronica.org/2011_3/28_bell/index.html

 

10. From the Children’s Discovery Museum of San Jose. Retrieved from;

https://www.cdm.org/mammothdiscovery/northamerica.html

 

11. Reppening, C. 1967 Palearctic-Nearctic Mamalian Dispersal in the Late Cenozoic. In The Bering Land Bridge Hopkins, D.M, ed. Stanford University Press.

 

12. Dr Jim Mead. 2015 A new HUGE discovery…and it is so OLD! From the Mammoth Site of Hot Springs South Dakota. Retrieved from:

http://mammothsite.com/a-new-huge-discoveryand-it-is-so-old-by-dr-jim-mead/

 

13. Meiri M et al. 2014 Faunal record identifies Bering isthmus conditions as constraint to end-Pleistocene migration to the New World. Proc. R. Soc. B 281: 20132167.

http://dx.doi.org/10.1098/rspb.2013.2167

 

14. The Bering Strait, Rapid Climate Change, and Land Bridge Paleoecology Final Report of the JOI/USSSP/IARC Workshop Held in Fairbanks, Alaska on June 20-22, 2005. Eds. Fowell, S and D. Scholl (Stanford University and USGS)

http://usssp-iodp.org/wp-content/uploads/Workshop_Bering-Strait_Report.pdf

15. Marine Isotope Stage;

https://en.wikipedia.org/wiki/Marine_isotope_stage

 

16. Lambek et. al. 2002. Links between climate and sea levels for the past three million years. Nature VOL 419 p199-206

 

17. M. Siddall, J. Chappell, E.-K. Potter, Eustatic Sea Level During Past Interglacials, in The Climate of Past Interglacials F. Sirocko, M. Claussen, T. Litt and M.F. Sanchez-Goni, Eds Elsevier 2006.,

 

18. Compton, John S. 2011. Pleistocene sea-level fluctuations and human evolution on the southern coastal plain of South Africa. Quaternary Science Reviews 30 506-527

 

The Peopling of the Americas I: Beringian Land bridges, Formation and Paleoecology

The peopling of the Americas has caused spirited and sometimes heated academic debate among scientists in the USA for decades.

For centuries the route of entry has generally been accepted as across the Bering Land Bridge.

The first writer to suggest this possible route of the entry of people into the American continent was a Jesuit monk named Jose de Acosta. De Acosta had arrived on the Caribbean coast of South America at Carthagena and travelled onwards to the settlement of Nombre de Dios. From here he journeyed through 18 leagues of tropical forest via the route pioneered by Balboa and De Avila to Panama. Once on the Pacific he took ship once more and reached Lima in the recently conquered Peru. It was while travelling to his new posting that he began recording his experiences and observations of fauna, flora, peoples and geography of the Americas. As an educated European he had read the extant writings of the ancient philosophers and upon crossing the equator he later related “He expected, as professed by the philosophers that he had studied, an unbearable intense heat in crossing the equator, but found it to be so cool in March, that he laughed at Aristotle and his philosophy”¹

De Acosta spent many years in Peru carrying out missionary work and lecturing on theology. Initially assisting the Viceroy, Don Francisco de Toledo he travelled the country for two years gaining a grounding in geography of the country. Subsequently he lived at Juli, on the shores of Lake Titicaca the main seat of the Jesuit order in Peru. His missionary duties required him to travel widely and thus he became intimately acquainted with the native peoples and their customs.

One theological problem that was therefore at the forefront of de Acosta's mind was a basic one: scriptures clearly defined man's origins as being in the Old World, therefore theologians must account for a branch of humanity in the New World. De Acosta phrases it thus:

 “Scripture teaches us that all men are descended from a first man. Hence man must have come to the Indies (The Americas) from Europe, Asia or Africa, but such pronouncements do not tell us the manner of travel or the route. ..How could the human race have traversed so great an immensity of seas and lands? How could so large a number of people have been concealed for so many centuries?”

De Acosta goes on to logically rule out intended voyages using navigation techniques as no evidence of such abilities used by ancient peoples exists in the written or oral traditions of the three continents known up to the discovery of the Americas. He discounts accidental voyages as the great diversity of animal life in the New World also had to arrive there in some manner, and in the case of terrestrial animals this had to be via a land connection with some continent.

He therefore goes on to conclude that the Indians and likewise the animals found in the New World probably arrived via some point in the far north at which the two worlds are either connected or closely approximate. He speculated that the Old and New Worlds in the far north either, “continued and joined with the other” or “they approach on nearer unto another”¹ such that the gap would not greatly hinder migration of animals or man.

In 1587, after 16 years in the Peru and later, Mexico de Acosta returned to Spain and occupied various prestigious offices, including lastly rector of the college at Salamanca. He published three great works based on the extensive manuscripts he had compiled during his time abroad. These were: De Natura Novi Orbis, De promulgatione Evangelii apud Barbaros, sive De Procuranda Indorum salute and above all, the Historia natural y moral de las Indias which contains his theory of the man’s entry into the New World.

Photo credit: Google Digital Commons
















The influence of these works was so profound that de Acosta became known as “the Pliny of the Americas”. Thus the land route for the peopling of the Americas entered the intellectual cannon almost from the beginnings of written history of science.
 
This close approach of one land mass to the other was first discovered by Semyon Dezhnyov in 1648 but news of the confirmed close proximity of the Asian and American continents only reached Europe 80 years later when Danish-born Russian navigator Vitus Bering entered the strait, now named after him, in 1728.
 
The theory remained in the realms of theological speculation until late in the 19^th century, when Wallace² suggested that the shallow seas separating Siberia and Alaska, specifically the area around the Bering Strait seemed the most likely site of geologically recent Eurasian-American land connections. In 1898 Wallace³, based on  water depth soundings of Bering Sea, Chukchi Sea and Being Strait, wrote that, “in later geologic times, more than once and perhaps during prolonged periods [there existed] a wide terrestrial plain connecting North America and Asia” .
The timing of the Bering Land Bridge formation remained a thorny issue for scientists with best estimates based on the synchronicity of Asian and North American, fossil, mammal faunas as detailed in the works of Osborn⁴ Willis⁵ and Simpson⁶.
The mechanism of Bering Land Bridge formation was initially thought to be related to tectonic events, however in 1934 Daly⁷ brought forward the theory that sea levels fluctuated drastically during the Ice Ages in response to the amount of water stored on land as continental glaciers such as the Laurentide of North America. Finally the area of the land connection was given its name; Beringia or the Bering Land Bridge by Hulten⁸ in 1937. His work noted that while large areas of North America and Southern Siberia were glaciated, large areas to the north of these remained ice-free refugia for many boreal plant species and animals and that the two continents were connected by the Bering land Bridge for a considerably length of time.
Thus the timing and existence of the Bering Land Bridge while firmly based in the, then current scientific evidence, required unequivocal data to fully prove the theory. With the intervention of World War Two the uncovering of definitive, confirmatory evidence had to await the emergence of a new generation of scientists working in the region.
 
One such scientist was David Moody Hopkins. After graduating from the University of New Hampshire in the summer of 1942 with a degree in Geology, he enrolled as a doctoral student at Harvard. Only three weeks into term-time he was offered a job with the U.S. Geological survey in Alaska locating strategic minerals for the war effort. His advisors at Harvard recommended that he take the post and finish his studies later, after al,l at 20 years of age Hopkins was of prime draft age. He thus accepted the job and was at once posted to Alaska. Through the autumn of 1942 and the summer of 1943 Hopkins mapped the distribution of molybdenum and coal deposits but also found time to collect Pleistocene fossils or marvel at, and study the glaciers and mountain scenery around his camps at Muir Inlet and in the Matanuska mountains. Each autumn it was back to Washington D.C. to write up his reports. In late 1944 he chose to write up his reports in Anchorage, Alaska hoping to drafted in Alaska and not back east. Thus it was that he was drafted whilst still in Alaska and did his basic training there. On completion of the course intelligence and aptitude tests were carried on the recruits, by the army to see which assignments they were best suited for. Experience was also a factor. Hopkins thus gave his experience working for the USGS, fictionalised teenage work on the railroad in New Hampshire and lastly experience of meteorological observations based on a class on climatology he had taken at UNH and watching a friend take such measurements. Basically it was an ‘Anything but the Infantry’ strategy. The army took one look at Hopkins’ weatherman experience and posted him 600 miles north to the extremely stormy cold bay, at the base of the Aluetian chain. Here Hopkins spent the rest of the war carrying out meteorological observations and using Hulten’s Flora of the Aluetian Islands to identify and collect plants. He was finally discharged in the spring of 1946 in the wake of the eruption of Okmuk Caldera on Umak Island.
Consequently, he returned safe and sound to Harvard to complete his Ph. D. He studied under Kirk Bryan an early proponent of the multidisciplinary approach to Geology. This approach was certainly absorbed by Hopkins. It was this combined with his experiences in Alaska before and during the war that formed Hopkins approach to Geology, in that he was prepared to take evidence from a wide field of knowledge and synthesise it into new explanations of natural phenomena. It was then that Hopkins began to see himself as more of a paleonaturalist blending the geology of landscape formation and his naturalist interests of early youth. This combination of interests, led him to be one of the first doctoral students to achieve a Ph. D from Harvard in Quarternary Geology.
 
Going back to work for the USGS Hopkins was able to pursue his interest in the Quartenary Geology of Alaska and in particular the history of the cyclic formation and inundation of the Bering land Bridge. By 1948 he was back in the field, in Alaska with a hand-picked team focussing on the subject that would become his life’s work.

Hopkins at Hammum Creek near the Imuruk River 1948.
Photo Credit O’Neill¹⁰

Sigafoos, Quay and Hopkins drying their gear at Lava Lake camp 1948
Photo Credit O’Neill¹⁰

Through the late 1940’s and 1950’s Hopkins never missed a season in Alaska, despite the tragically early death of his first wife, a disastrous second marriage and being wed again for a third time! He often based himself around the Seward peninsula and worked with a range of scientist or visited others working in the region such as Louis Giddings and Bob Sigafoos. In 1959, Hopkins‘⁹ long years of work led him to publish his first major paper on the Bering land bridge. In it he theorised about the first formation of a land bridge in the Miocene, its first inundation at the Miocene-Pliocene boundary and its continued cyclic formation and submergence during the Pleistocene. Casting his mind back to that time Hopkins comments¹⁰ that “..it suddenly occurred to me that if we three got together we could perhaps solve the problem of the Bering Land Bridge. We could show whether it existed or not and when it existed or not .. The Land Bridge was in the air. It had been for years.”
Whilst not all of his peers accepted these theories, he was fast becoming the best known and widely published figure in Bering land Bridge research.
Thus it may not seem surprising, that in 1963 he was asked by the International  Association for Quarternary Research to organise an all-day symposium on the land bridge for the 1965 INQUA conference.
This galvanised him to begin reaching out to Russian scientists such as Merklin and Petrov and others to request paper proposals for the upcoming symposium. The back and forth of distributing drafts to symposium participants, the insistence that contributors consider and comment on others’ work and address it in print before the symposium, and the judicious editing of the papers ended up with the event being one of the best organised and productive ever organised. It has since been held up as an example of best practice in organising such events. In the year of the symposium it also led Hopkins to jointly publish 1965 published a paper with Merklin and Petrov and other U.S. scientists¹¹.
The work however had only just begun. Now the symposium and it’s papers needed to be organised and published for their full import to be realised. Hopkins set to work with relish. After two years he had edited the monumental tome “The Bering Land Bridge”. It was published in 1967. Hopkins of course, personally wrote the summary chapter¹².  

 

Hopkins on publication day.

Photo Credit O’Neill¹⁰

 

 

Cover of the ‘Bible’ as colleagues began to call the Bering Land Bridge.

Photo credit N Barden

 

The results presented from many different disciplines and by scientist worldwide and their subsequent analysis and synthesis by Hopkins represented a huge step forward in the understanding of the Bering Land Bridge. In summary the results were:

 

·         During the Pleistocene Era (2 million years ago to 10 000 years ago), Beringia was a large, ice-free land mass during periods of glaciation

·         As the climate warmed and the glaciers melted, the Bering-Chukchi platform would flood, severing the terrestrial connection between North America and Asia. Eventually, the climate cooled and glaciers reformed, exposing the Bering Land Bridge again.

·         When exposed, this land bridge was covered with vegetation and supported a variety of animal life

·         That 4 waves of mammalian immigration had occurred over the land bridge, once during early Miocene, again at the Pliocene-Pleistocene boundary and probably at least twice more during Pleistocene. Furthermore the climate on the land bridge appeared to be temperate, humid and forested in the first two episodes, temperate grasslands in the mid Pleistocene and only in late Pleistocene was the climate significantly colder being represented by tundra, steppe and Taiga plant communities. Additionally there was some back migration from the Americas to Asia.

·         That the land bridge had existed at least nine times during the Pleistocene

 

Table by Reppening showing envisaged mammalian migrations during the late Pliocene and Pleistocene¹³

 

Whilst a huge amount of the work was accepted by the scientific community at large many questions remained for Hopkins. For example was it possible to reconstruct the ancient climate on and near the land bridge and more particularly could he reconstruct the climate during the LAST land bridge. For this it was assumed would allow basic questions about the peopling of the Americas to be, at last, definitively answered.

 

During the editing of The Bering land Bridge in 1966, Hopkins did not forsake his usual Alaskan, summer field season. Whilst camped on the north shore of the Seward peninsula, Dale Guthrie and John Matthews turned up to study fossil insects in sedimentary deposits that Hopkins had previously recommended to them at the adjacent, Cape Deceit on Kotzebue Sound. As Hopkins says¹⁰: “I had reason to think that Cape deceit had a record that went at least back through the last glaciation, the IIlinoian. I had no idea that it would go back, possibly, to the Pliocene.. They started digging at Cape Deceit, which turned out to be one of the most important contributions ever to Beringian Paleoecology.” The Pliocene/Pleistocene boundary fossil faunal were presented in The Bering land Bridge, a year later by Reppening¹³. It was not these fossils, impressive as they were, however, that were of key importance. It was the fossil pollen, plant fragments and body parts of assorted insects that were the real goldmine. For it was in these, that scientist could start to reconstruct the paleoclimate of the Bering Land Bridge. Their work indicated that the early Pleistocene climate in the region had been a dry, scantily clad Tundra which became grassland dominated in mid-Pleistocene and Steppe Tundra at the end of the geologic period and that in warmer interglacials treelines had been considerably further west, and in fact as far west as Cape Deceit, on at least two occasions. In other words there were considerable periods, during which, the migration of large herbivores such as mammoths and presumably their human hunters could have crossed the land bridge throughout the Pleistocene.

This view of the paleoecology of Beringia was not shared by all, however. Another of Hopkins’ associates Paul Colinvaux, had, at Hopkins’ suggestion had carried out sediment coring at Imuruk Lake. The view of the paleoecology of Beringia he drew from the fossil pollen record was in stark contrast to that of Guthrie and Matthews. He viewed Beringia as an  , a windy, forsaken landscape of sparse tundra, “a dusty plain stretching to the horizon, vegetated between the bare patches with a low mat of sedges and grasses looking like a drier version of the modern arctic plain.”¹⁴

Twelve years after the publication of his first book Hopkins arranged a second conference to discuss the current work of the Paleoecology of Beringia, the Wenner-Gren Foundation for Anthropological Research Symposium was held June 8-17, 1979 in Burg Wartenstein Austria. The resulting book¹⁵, The Paleoecology of Beringia was published in 1982.

 

The publication, caused, what has become the longest running argument in Beringian studies. Ritchie and Cwynar stated, in 1982¹⁸ that “We suggest that the ‘Arctic Steppe Biome’ never existed”. Also critical of the productive Mammoth Steppe theory was Collinvaux and West’s 1984¹⁴ view of the Beringian Paleoecology. This was swiftly followed in 1985¹⁶ by Guthrie with his provocatively titled “Woolly Arguments Against the Mammoth Steppe”. Colinvaux’s 1986 reply¹⁷ was equally biting. The situation became so bad that Guthrie is reputed to have dumped a huge, muddy, freshly dug mammoth tusk on Colinvaux’s desk and heatedly told him that he had dated Mammoth remains spanning the whole of the existence of the Bering land Bridge. Colinvaux was unimpressed and continued to insist (in print) that these skeletal remains were the result of migrations of mammoth from distant productive areas. Guthrie’s response “They have to eat every 12 hours!” and evidence in the form of the grass and herbaceous gut contents of mammoth remains sealed in permafrost from Siberia fell on deaf ears.  

 

What was needed was more evidence to demonstrate one way or another whether Beringia was a cold, unproductive arctic desert, or a rich Mammoth Steppe with herbaceous vegetation capable of supporting large herds of grazing animals. Unsurprisingly some of the evidence was actually and literally uncovered by Hopkins himself. In 1988, following up on a 1974 observation of what he believed to be a preserved Pleistocene ground surface, he took his graduate students Claudia Hofle and Victoria Goetcheus to the Devil Mountain-Cape Espenberg area. Here, sealed by a layer of volcanic tephra, they uncovered actual preserved remains of a rich array of herbaceous plants, shrubs and grasses. Furthermore, radiocarbon dating of the remains gave a date of 18000BP, i.e. in the middle of the period disputed by the Palynologists, Colinvaux and Ritchie on one hand and Guthrie and Young on the other. This supported the Mammoth Steppe theory. Later publications by Hofle^{23a and b} and Goetcheus^{24a and b} confirmed this.

 

Further evidence came to light from Cinq-Mars’ excavation of the Blue Fish Caves¹⁹ in the Yukon. Cinq-Mars states:

“More importantly, these results demonstrate that the mammoth steppe.. constituted an essential element of the Glacial Maximum biotope of eastern Beringia (between about 17,000 and 25,000 BP).”

Basically Cinq-Mars is saying that their palynological and other data show that Mammoth Steppe did exist at the time of the sediment formation over height of the last glacial maxima (ca. 22000BP). In other words their data refute the “productivity paradox” of Schweger et al.²⁰ (1982) and the position of Cwynar and Ritchie (1980)²¹ and support the position of Guthrie (1985)¹⁶ and Matthews (1982)²² that the environment was extremely rich and capable of supporting an extensive megafaunal assemblage.

 

Further publications, including Guthrie 2001²⁵ have thus stabilised the view that the Mammoth Steppe had actually existed as suggested and was capable supporting large Pleistocene grazing fauna which, in turn, were followed into the new world by humanity.

 

Here Hopkins’ direct involvement of the investigations of the Bering Land Bridge ends. He died aged 79, in November 2001.

 

Research has of course, continued. In fact Hopkins’ hypothesis in 1967 that the Bering Land Bridge had existed at least nine times has been updated by workers such as Pielou (1991) who theorises²⁶ that the Bering Land bridge has existed at least 20 times during the Pleistocene.

 

The Bering land Bridge at the height of the Illinoian glaciation. From Hopkins¹² p462

 

 

 

The Bering land Bridge at the height of the Wisconsin glaciation. From Hopkins¹² p462

 

References

1. Joseph de Acosta, Edward Grimston, Clements Robert Markham, The Natural and Moral Historie of the Indies, Hakluyt Society, 1880 pp.i-ii

 

2. Wallace, H. R. 1876. The geographical distribution of animals: harper, New York 2 vols.

 

3. Dawson, G. M. (1898). Geologic notes on some of the coasts and islands of Bering Sea and vicinity: Geol. Soc. American Bulletin., v.5 p117-146

 

4. Osborn, H. F. 1909. Cenozoic mammal horizons of western North America: US Geol. Survey Bull. 361 138p.

 

5. Willis, Bailey. 1909. Paleogeographic maps of North America: 13, Eocene-Oligocene North America: Jour. Geology v.17 p506-508; Quarternary North amerioca: Jour. Geology v17 p600-602

 

6. Simpson, G. G. 1947. Holarctic mammalian faunas and continental relationships during the Cenezoic: Geol. Soc. America Bull., v.58 p.613-687

 

7. Daly, R.A. 1934. The changing world of the Ice Age: Yale Univ. Press, 271p.

 

8. Hulten, Eric. 1937. Outline of the history of arctic and boreal biota during the Quarternary Period: Bokforlags Aktiebolaet Thule, Stockholm, 168 p.

 

9. Hopkins, D.M. 1959. Cenozoic history of the Bering Land Bridge. Science, 129, 1519–1528.

 

10. O’Neill, D. 2004. In The Last Giant of Beringia. Westview Press Boulder Colorado p. 107

 

11. Hopkins, D.M., MacNeil, F.S., Merklin, R.L. and Petrov OM (1965) Quaternary correlations across Bering Strait. Science, 147, 1107–1114.

 

12. Hopkins, D.M. 1967. The Cenozoic history of Beringia—A Synthesis., in The Bering Land Bridge Hopkins, D.M, ed. Stanford University Press.

 

13. Reppening, C. 1967 Palearctic-Nearctic Mamalian Dispersal in the Late Cenozoic. In The Bering Land Bridge Hopkins, D.M, ed. Stanford University Press.

 

14. Colinvaux, Paul A. and Frederick H. West. 1984. The Beringian Ecosystem. The Quarterly Review of Archaeology, Sept issue 1984.

 

15. Hopkins, D.M., Matthews, J.V., Schweger, C.E. and S.B. Matthews. 1982. The Paleoecology of Beringia. Wenner-Gren Foundation for Anthropological Research. Symposium. New York : Academic Press.

 

16. Guthrie, R. Dale. 1985. Woolly Arguments Against the Mammoth Steppe - A new look at the Palynological Data. The Quarterly Review of Archaeology, Sept 1984.

 

17. Colinvaux, Paul, A. 1986. Plain Thinking on Bering land Bridge Vegetation and Mammoth Populations. The Quarterly Review of Archaeology, March 1986. 

 

18. Ritchie, J.C. and L. Cwynar. 1982. “The Late Quaternary Vegetation of the North Yukon.” In: Paleoecology of Beringia, ed. D.M. Hopkins et al. (New York, Academic Press) pp. 113-126.

 

19. Cinq-Mars, Jacques and Richard E. Morlan. 1999. “Bluefish Caves and Old Crow Basin: A New Rapport,” in Ice Age Peoples of North America, ed. by Robson Bonnichsen and Karen L. Turnmire, pp. 200-212. Corvallis: Oregon State University Press for the Center for the Study of the First Americans.

 

20. Schweger, C.E., J.V. Mathews, Jr., D.M. Hopkins and S.B. Young (eds.) 1982.

“Paleoecology of Beringia – A Synthesis.” In: Paleoecology of Beringia, ed. D.M. Hopkins et al. (New York, Academic Press) pp. 425-444.

 

21. Cwynar, L. and J.C. Ritchie.1980. Arctic steppe-tundra: A Yukon perspective. Science Vol. 208, pp. 1375-1377.

 

22. Matthews, J.V., Jr. 1982. East Beringia during Late Wisconsin Time: A Review of the Biotic Evidence. In: Paleoecology of Beringia, ed. D.M. Hopkins et al. (New York, Academic Press) pp. 127-150.

 

23a. Hoefle, C. L. Buried Soils on Seward Peninsula, Northwest of the Bering Land Bridge. Masters thesis, University of Alaska Fairbanks, August 1995.

 

23b. C. Hoefle, C. L. Ping, D. Mann and M. Edwards

Buried Soils on Seward Peninsula: A Window into the Paleoenvironment of the Bering Land Bridge. Current Research in the Pleistocene Vol. 11, 1994.

 

24a. Goetcheus, Victoria G. 1995. The vegetation of A 17 Buried Surface on the Northern Seward Peninsula. 24^th Artic Workshop, Quebec. Abstracts, 1995.

 

24b. V. G. Goetcheus, D. M. Hopkins, M. E. Edwards and D. H. Mann

Window on the Bering Land Bridge: A 17,000-year-old Paleosurface on the Seward Peninsula, Alaska Current Research in the Pleistocene Vol. 11, 1994

 

25. Guthrie, Dale. R. 2001. Origin and causes of the mammoth steppe: a story of cloud cover, woolly mammal tooth pits, buckles, and inside-out Beringia

Quaternary Science Reviews 20 ps.549-574

 

26. Pielou, E.C. 1991.  After the Ice Age: The Return of Life to Glaciated North America. University of Chicago Press, Chicago