Siberian Archaeology 2: Diring Yuriakh

The Diring Yuriakh site lies approximately 140km SSW of Yakutsk on the Lena River, on a point formed by its junction with its tributary, the Diring Yuriakh creek.

 

 

Diring Yuriakh location. Photo credit: Walters 1997 (1).

Original caption: Fig. 1. Map of eastern Russia showing the location of Diring Yuriakh.

 

Approaching the Diring Yuriakh site along the Lena River (3).

Photo credit: Courtesy of the Center for the Study of the First Americans.

The Diring Yuriakh site was discovered accidentally, in June 1982, by Russian geologists, led by O. V. Grinenko whilst they were excavating a geological test pit to show the Cenezoic deposits of the 90m Tustakhsk terrace of the Lena River to participants in the International Union for Quaternary Research, (INQUA) congress XI held in Moscow and came across human bones (2).

The human bones recovered were of late Neolithic date and proved to be from a series of burials or graveyard belonging to the Ymyiakhtakh culture as shown by an excavation carried out in October of the same year, by Yuri Mochanov.

 

Excavations at Diring, adapted from Mochanov 1993 (2)

Yuri Mochanov

Photo credit: Courtesy of the Center for the Study of the First Americans (3).

The history of the site discovery and subsequent excavations is recounted by Mochanov in his first English language publication on the site The Ancient Palaeolithic Site of Diring and the Problem of a Nontropical Origin for Humankind (2). Most of what follows is a summary of that paper:

 

“Near burial V at a depth of 60cm, a flint core, seven flint microblades and 14 small flint flakes were found in the second paleosol from the bottom.. and belong to the late Neolithic Diuktai culture.. in the lower paleosol a concentration of quartzite artifacts was found lying directly on top of the gravels. These included an anvil cobble, two cobble hammerstones, eight amorphous flakes and 92 pieces of debitage.. The concentration of quartzite artifacts was separated from the Diuktai finds by a layer of sterile sand 5-12cm thick. The absence of any kind of diagnostic tool prevented the clear establishment of a cultural-chronological relationship for the quartzite artifacts. In appearance they were unusual for the early cultures of the Yakutia (the region of the site). ..the flakes were obtained (from cores) by a nonsystematic method of splitting cobbles ..unlike the Diuktai flint artifacts, there were well-preserved traces aeolian abrasion on many of the quartzite artifacts ..Traces of abrasion were present not only on pebble cortex, but on the cleavage plane of the detached flakes and debitage as well. ..I have not in 20 years of work on archaeological sites of Northeast Asia, encountered either the technique of nonsystematic flaking of cobbles or wind-abraded stone artifacts. ..(previous) experiments in the flaking of cobbles and nodules of various stone and examination of various museum collections of stone tools permitted the following conclusion: the method of nonsystematic flaking (of cobbles and nodules) was characteristic only during the earliest stage of the Stone Age, when people had not yet mastered the techniques of removing flakes from specially prepared cores..”

 

Flaked Cobbles and Chopper (3).

Photo credits: Courtesy of the Center for the Study of the First Americans.

 

 

 

Core with flakes (above) and flakes refitted (below). (3).

Photo credit: Courtesy of the Center for the Study of the First Americans

In a roundabout manner Mochanov then brings up the technologically similar, simple nonsystematic breaking of stones found in the Oldowan and Acheulian lithic complexes of approximately 2.5 to 1.5Mya from Africa.

 

However he goes on to caution that the site investigation at that stage was far, far from complete and would need a huge amount of systematic work, particularly of stratigraphy to draw any firm conclusions.

 

In 1983, Mochanov was back excavating again. He continues his description of the excavations, noting that by the end of this season 1300 quartzite artifacts of the same type found in the previous year had been found and that the site had expanded to cover 3000m². Some further commentary follows, reinforcing his belief that the site’s lithic assemblage resembled the Oldowan industry of Africa. He notes the discrepancy between the PAE (Prelinsk Archaeological Expedition) view of the site stratigraphy, that aeolian deposits overlay the cultural stratum and his assumptions of great age (dependent on then layers above the cultural bearing stratum 5 being of alluvial origin)  and decides more work needs doing on the site stratigraphy. Here is his description of that stratigraphy from 1983:

 

“In 1983 it was determined that the excavations, trenches, and test pits that the wind abraded quartzite artifacts on the 105-120m terrace were derived from the gravel-pebble-cobble deflated layer which was lying on the red-coloured fluvial sands..

(Deflation is defined as “The removal of surficial deposits of soil, sand or fine gravel by wind action.”) On a bedrock foundation of Cambrian limestone at an elevation of 105m +/- 2m.. It was observed that the culture-bearing stratum intersects the polygonal, sand filled ice-wedges, extends into the fluvial sands at 109-120m, and also intersects the lower gravels at 105-109m and in some places the eluvial top of the bedrock terrace.

In several places on the 105-120m terrace, chiefly at elevations greater than 108m, horizontally layered sandy deposits with cryogenic syngenetic textures and structures were recorded above the culture-bearing stratum and were assigned by us to the floodplain facies of the alluvium. Its thickness varied from 1 to 3.5m. The alluvial, owing to postdepositional denudation in some areas, mostly at the outer bench of the terrace, were marginally preserved or had lost their texture and structure, but in other areas they occurred as distinct ridges separated by furrows... The leaders of the A-14 (geological expedition mention above) had taken it (the sandy layers above the culture bearing stratum) for a dune and evidently therefore decided that its deposition was due to aeoliain processes. They adhered to this point of view until 1985. However in 1983 the PAE (Prelinsk Archaeological Expedition) had clearly noted that at the basal edge of the 105-120m terrace, the horizontally layered sands which covered the culture-bearing stratum were deposited as a sheet wash over the loamy-clay deposits of the 125-135m terrace, and for this reason they could not be aeolian.”

 

This is a key point in Mochanov’s argument. If the deposits above the cultural-bearing stratum are alluvial and not aeolian in origin, then the stratigraphy is intact and therefore the artifacts are EXTREMELY old as he has assumed all along. Mochanov concludes:

“Based on the geomorphological development of the middle Lena terrace, the 105-120m terrace was determined to be Cherendeisk or older, ..which contain the culture-bearing stratum - was restricted to the range of 2.4-1.5 million years.”

 

Here is a schematic view of the stratigraphy from Mochanov’s paper (2):

Mochanov’s schematic of the stratigraphy adapted from his 1993 (2) paper, note I have added a red line to indicate the cultural bearing layer, just above the v-shapes of the ice wedges. Original caption reads: Fig. 2. A. Schematic section of the Tabaginsk terrace in the vicinity of the Diring site. (a) Kembriisk (Cambrian) limestone; (b) gravel stratum2; (c) sand of stratum 3; (d) sandy Ice Wedges; (e) deflated culture bearing gravels; (f) sands, sandy loams and loams of strata 6-10; (g) sandy loams and loams of stratum 11; (h) sandy loams and loams of stratum 13; (i) sands of strata 12 and 14-18; (j) cultural remains of the Most Ancient Paleolithic; (k) drill cores; (l) numbers of the strata. B. (inset) Schematic profile of the locale showing the bedrock foundation of the terraces in the vicinity of the Diring site (not to scale).

 

In 1984 Mochanov was able to acquire more resources, in the form of a bulldozer, to test the extent of the cultural layers extending away from the river and under the higher terrace levels. During that season (2) and subsequently (4) 15850m² of sod was removed, and an area of excavations of 10220m² was excavated, of which 7743m² was the cultural bearing layer. In total 40m of overburden was removed from the centre of the site. By 1988 28 lithic clusters numbering thousands of artifacts had been recovered (4).

 

 

Diring Yuriakh upper site with overburden removed. Note the extreme depth of sediments removed at left of photograph.

Photo credit: Courtesy of the Center for the Study of the First Americans (3).

During the next part of the paper, Mochanov then theorises at length about the whole geological chronology of the Diring Yuriakh-Lena River watershed. Whilst interesting it has no bearing on the overall dating of the stratum 5, the cultural-bearing stratum.

He then to rebuts the views of Medvedev that “for wind abrasion (of the type seen at Diring Yuriakh) of stone to occur, severe artic desert conditions with continuous high velocity - more than 70m/s - were necessary” by detailing the palynological evidence that, pine, larch, fir, birch, bird cherry, alder and mountain ash grew in the region at the time and thus preclude the area being an artic desert. He further discounts this scenario by fairly pointing out that, if winds of this ferocity has scoured the site, the tiny pieces of flake debitage would now not remain in situ as they were indeed found.

 

He then attempts to justify ignoring the opinions of geologists Alexeev and Kamaletdinov that the stratigraphy has ..“inconvenient geological layers related to lake, slope and aeolian deposits” and their judgement that the maximum age down to the bedrock to be no more than a million years.

 

In summary Mochanov sees the entire deposition sequence above the bedrock, as being attributable to the fluvial/interfluvial cycle of the Lena River. Furthermore he sees the cultural-bearing layer as being formed, eroded, the artifacts being abraded by windblown sand, then re-buried by further fluvial/interfluvial cycles of the Lena River. Thus the dates of this layer MUST, according to his theory, be between 2.5 and 1.5 Mya. He notably chooses to ignore the opinions of the geologists that the lithic artifact-baring stratum has been buried by a layer of sediments which are of aeolian origin.

 

Of more interest is the single suspiciously (human?) fractured mammal bone of ungulate or proboscid origin. It was found in a concentration of lithic artifacts and is of obvious antiquity being entirely mineralised.

 

The lithics themselves are quite remarkable in their crudity and their obvious human manufacture.

 

Mochanov gives the following circumstances as indicating their human provenance:

·         The lithics are found in concentrations.

·         Lithics not randomly scattered across the landscape

·         Lithics found at maximum distances of 0.5Km from each other

·         Reconstruction of cores was possible using flake debitage

 

Characterising the concentrations of lithics, Mochanov notes that vast majority were of quartzite with a very low number of sandstone-quartzite artifacts also present. The concentrations of a few hundred artifacts occupied areas of about 100m². A typical assemblage of artifacts as for example from concentration 14 consisted of 18 kinds of ordinary choppers, multiedged macrochoppers one sharpened end microchopper (including one with a bill), one lateral microchopper, an example of the uniquely Siberian Skreblo -  a wide oval scraper, variously poorly formed tools, possibly resembling scrapers, knives, points or burins. Lastly a number of hammerstones and anvils. There was also considerable flake debitage, all the above grouped round a heavy cobble-anvil of dimensions 42 x 31 x 30cm.

Anvil on lag-surface

Photo credit: Courtesy of the Center for the Study of the First Americans (3).

Mochanov then freely admits just how basic and depauparate the assemblage is: “The basic distinction of the Diring Complex from the earliest African complex is the fact that at Diring multifaceted spheroids, bifacial discs, clearly expressed chopping tools, proto-axes, well-retouched small tools on chunks and flakes and above all, unmistakeable cores are lacking.”

Of final note in this paper is the claim that work by A. V. Pen’kov established a range of dates based on the paleomagnetic reversals above and below stratum 5 (the cultural-bearing layer). These ranged between “4.2-3.9 My (stratum 3), 3.15-3.0My (stratum 6, 7 and 8). A variation is possible as well 3.4-2.9 or 2.5 My. At present a “minimum” variation also cannot be excluded: 19.-1.7 My.”

 

 

What on Earth are we to make of all this? Oldowan type stone tools at an impossibly early date way out there in Siberia of all places? And the lead investigator at odds with his colleagues (I have only hinted at that - the detailed story shows far greater depths of disagreement)?

And yet.. and yet those tools are so obviously formed for human purposes.. the only real question is can we get REAL dates and then perhaps decide on the really juicy bit.. The WHO.. the which species of human!

 

 

Luckily for us someone did step up to try to bring some cold hard science to the site.. and luckily for us it was a well-respected outsider.. none other than Michael Waters of Buttermilk Creek fame (see here).

Here is how Waters (1) explains the stratigraphy of the site:

“At Diring, unconsolidated Quaternary sediments rest unconformably on Cambrian-age limestone. These Quaternary sediments are divided into four major stratigraphic units labeled I through IV (from oldest to youngest) and are further subdivided on the basis of lithostratigraphic criteria (Fig. 2). These sediments are of alluvial and eolian origin. The oldest unit that overlies limestone bedrock is composed of well-rounded gravel (unit Ia). Most of the gravels are pebble-sized and composed of quartzite. The gravels are conformably overlain by sands (unit Ib). The sand ranges from coarse-to-fine, angular-to-subangular grains that occur in horizontal beds and crossbeds. A few thin beds of well-rounded fine gravel and granules are interbedded in the sand. Unit I represents fluvial deposition in a sandy braided channel of an ancestral Lena River.

The gravel and sand (units Ia and Ib) are cut by two sets of wedges (3, 5) filled with sand (unit II). Wedges in the first set are large, ranging from 0.6 to 5 m wide and 4 m deep. Wedges in the second set are less than 0.5 m wide and reach a maximum depth of 1.1 m. All the wedges are filled with well-sorted, subangular medium sand. In some cases, small gravels (0.5 to 1 cm in diameter) form distinct vertical beds in the sand wedge fill. Large pebbles or cobbles are absent from both wedge sets. The sand from unit II is well sorted and the grains show evidence of wind abrasion; thus, the sand appears to be of eolian origin. The larger wedges appear to have been truncated by later deflation, whereas the smaller wedges extend downward from the deflation surface. Resting on top of the eroded surface of unit Ib and the truncated sand wedges (unit II) is a gravel lag. This is a loose lag with mostly small pebbles (92%), a few large pebbles (7%), and rare boulders (1%). Compositionally, most of the gravels are siliceous (70%), with the remainder composed of quartzite (20%) and vein quartz (10%). Most of the larger pebbles, and all cobbles and boulders, are quartzite. These quartzite clasts are generally well rounded.

All gravels are wind-abraded, showing pits, facets, and polish. This lag appears to have been created by the eolian deflation of unit I. The upper part of unit I must have contained lenses of gravel and large boulders that were deflated to a common surface and concentrated into a loose lag as the fine-grained sediments were removed when the wind swept over the area. The artifacts from Diring are found on this deflation lag.”

So an extremely simple stratigraphy then: the cultural-bearing stratum in an aeolian deflation lag and NOT of alluvial origin as Mochanov had assumed. In fact it was exactly what his own geologists had told him it was in 1983!

Walters 1997 Fig 2.(1) Original caption: Generalized cross section of the stratigraphy at Diring Yuriakh and associated TL ages. Solid triangles indicate the positions of the artifacts.

As can be seen from the diagram Thermoluminescence (TL) dating was used to date the sediments. Here is how Walters explains the method and his results:

“Thermoluminescence (TL) dating of sediments was the only acceptable technique available to assess the age of the stratigraphy and artifact horizon at Diring because of the proposed antiquity of the site and the absence of materials suitable for other dating methods. Loess and cover sands at Diring Yuriakh are suitable for TL dating because these sediments presumably received prolonged light exposure during subaerial eolian transport and deposition.

This process resets the TL signal to a low definable level. As a test, we exposed the sediment from Diring to sunlight for 16 hours. As a result, natural TL emissions were substantially reduced by .84%. A slightly greater reduction in TL occurred after 8 hours of exposure to an ultraviolet (UV) light–dominated source, which approximates sunlight exposure for .24 hours and provides a better estimate of the full solar resetting level. The 8-hour UV exposure values were used to calculate the TL ages reported here because they provide a maximum estimate of the predepositional TL level and a finite estimate on eolian deposition. The fine-grained (4 to 11 mm) feldspar-dominated fraction was used for dating because of the greater likelihood of solar resetting of the grains and because the grains are ubiquitous in the stratigraphic sequence. The paleodose was determined by the total-bleach technique; there was no discernable instability in the laboratory- induced emission. An attribute of the sediments from Diring that contributes to rendering TL ages >100 ka is the uniform and relatively low dose rate for loess and eolian sand units at 3.7 to 4.0 grays (Gy)/ka and 3.6 to 2.8 Gy/ka, respectively. We obtained nine TL ages at Diring. Fine-grained polymineralic samples from units IIIa and II (large truncated wedge) that bound the artifact-bearing horizon responded sufficiently to laboratory addititive dose and yielded ages of 267 +/- 24 ka (sample OTL471) and 366 +/- 32 ka (OTL472), respectively. .. The similarity among TL ages on polymineral samples and quartz grains, despite different levels of luminescence emissions, indicates that TL ages reflect burial time and are not an artifact produced by combining TL signals of various apparent ages. .. TL ages from the loess of unit IIIe that overlies the archaeological material at Diring provides a minimum age estimate for the artifact-bearing surface of about 260 ka, whereas the TL age from unit II underlying the artifact surface provides a maximum age of about 370 ka. The artifact surface, then, may date to about 300 ka.”

 

Whilst there has been some criticism of Walters’ TL methodology over the years, generally the sites’ antiquity is now well established.

 

Verdict:

 

Site inhabited ca. 300, 000 years ago.

 

Likely candidates Homo erectus or Homo heidelbergensis

 

 

References

1. Michael R. Waters, Steven L. Forman and James M. Pierson. 1997. Diring Yuriakh: A Lower Paleolithic Site in Central Siberia. Science Vol. 275 pp. 1281-1284. DOI: 10.1126/science.275.5304.1281

2. Mochanov, Y. A. 1993. The Ancient Palaeolithic Site of Diring and the Problem of a Nontropical Origin for Humankind. Arctic Anthropology vol. 30, no 1 pp22-53.

 3. Center for the Study of the First Americans. Retrieved from: http://csfa.tamu.edu/gallery.php

4. Carlson, R.L. 2001. Diring Yuriakh: An Early Paleolithic Site on the Lena River, Eastern Siberia. Indo-Pacific Prehistory Bulletin 21, (Melaka Paers vol. 5)







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