New DNA Analysis Allows for Greater Understanding of Modern Human Prehistory

From Science Daily:

A University of Utah-led team developed a new method for analyzing DNA sequence data to reconstruct the early history of the archaic human populations. They revealed an evolutionary story that contradicts conventional wisdom about modern humans, Neanderthals and Denisovans.

The study¹ found that the Neanderthal-Denisovan lineage nearly went extinct after separating from modern humans. Just 300 generations later, Neanderthals and Denisovans diverged from each other around 744,000 years ago. Then, the global Neanderthal population grew to tens of thousands of individuals living in fragmented, isolated populations scattered across Eurasia.

"This hypothesis is against conventional wisdom, but it makes more sense than the conventional wisdom." said Alan Rogers, professor in the Department of Anthropology and lead author of the study that will publish online on August 7, 2017 in the Proceedings of the National Academy of Sciences.

As the authors note, there is mounting evidence of a migration from Africa to Europe, likely by way of the Strait of Gibraltar, in which Acheulean stone tools are introduced by a group which became known as Homo heidelbergensis.  This group, subsequently, split, likely gave rise to the Neandertals  and the Denisovans a bit later.  Then, a second wave of migrations around 90 to 100 ky happened, during which there was hybridization between Neandertals, moderns and Denisovans.

We live in a time where we now have the genomes of archaic Africans, Eurasians, Neandertals, Denisovans and early moderns and can compare them using site pattern analysis.  These authors found that

• In contrast to previous studies, this one indicates that Neandertal population size was large, on the order of tens of thousands of people, in isolated populations
• The split between the Neandertals and Denisovans was early, on the order of 744 thousand years ago.  
• By the time you find these hominins in the fossil record, they had already split from the Denisovans, which explains why the Atapuerca Sima De Los Huesos hominins resemble Neandertals more than Denisovans.
• Therefore, Neandertal features are thought to have emerged over a long period of time in Europe. 

This analysis will help us rethink our understanding of the fossil material from Gran Dolina, which now likely represents a population from which both the Denisovans and Neandertals split.  This makes sense, given the general, non-derived nature of the fossil material from there.  Things are slowly (maybe?) falling into place.

¹Alan R. Rogers, Ryan J. Bohlender, and Chad D. Huff. Early history of Neanderthals and Denisovans. PNAS, August 2017 DOI: 10.1073/pnas.1706426114

Nuclear DNA From the Simo de los Huesos Fossil Material

Nature has a paper that focuses on DNA that has been extracted from the Sima de los Huesos fossil material from the site of Atapuerca, in northern Spain.  The letter does not appear to be behind the pay wall, at the moment.  In 2014, researchers completed a mitochondrial genome sequence for a hominin and the results were interesting, to say the least.  Here is the abstract from the 2014 paper:

Excavations of a complex of caves in the Sierra de Atapuerca in northern Spain have unearthed hominin fossils that range in age from the early Pleistocene to the Holocene¹. One of these sites, the ‘Sima de los Huesos’ (‘pit of bones’), has yielded the world’s largest assemblage of Middle Pleistocene hominin fossils^{2, 3}, consisting of at least 28 individuals⁴ dated to over 300,000 years ago⁵. The skeletal remains share a number of morphological features with fossils classified as Homo heidelbergensis and also display distinct Neanderthal-derived traits^{6, 7, 8}. Here we determine an almost complete mitochondrial genome sequence of a hominin from Sima de los Huesos and show that it is closely related to the lineage leading to mitochondrial genomes of Denisovans^{9, 10}, an eastern Eurasian sister group to Neanderthals. Our results pave the way for DNA research on hominins from the Middle Pleistocene.

The authors argued at the time that Denisovans and the population from Sima de los Huesos shared a common ancestor, with a split of about 700 kya, and that both are more closely related to each other than either is to Neandertals.  This strongly suggests that Neandertals are a later derivation from a common crown group that represented the progenitors of all three groups.  The authors suggest that these findings are unexpected because of the incipient Neandertal traits found in the Sima collection, including occipital bunning in the back of the head and some mid-facial prognathism (it looks like the maxilla is puffed out).  Arsuaga and others have stated previously that they thought all of the Sima de los Huesos material belonged to the Neandertal lineage. That, apparently, is not the case.  As these authors point out, there may have been more than one evolutionary lineage roaming the valleys and forests of Europe.

Now the nuclear DNA has been  sequenced and these results strongly suggest that the people represented by the SH population were, in fact, early Neandertals.  From the paper by Meyer et al:

The nuclear DNA sequences of femur AT-5431 and the incisor show that they belonged to the Neanderthal evolutionary lineage, and the limited data available for the molar suggest that the same is true for this specimen. Thus, the results show that the SH hominins were early Neanderthals or closely related to the ancestors of Neanderthals after the divergence from a common ancestor shared with Denisovans. Although it is difficult to determine the age of Middle Pleistocene sites with certainty, geological dating methods¹, as well as the length of the branches in trees relating the mtDNAs from femur XIII and an SH cave bear to other mtDNAs^{2, 12}, suggest an age of around 400,000 years for the SH fossils. This age is compatible with the population split time of 381,000–473,000 years ago estimated for Neanderthals and Denisovans on the basis of their nuclear genome sequences and using the human mutation rate of 0.5 × 10⁻⁹ per base pair per year⁷

These authors now suggest that, in combination with the MtDNA results from last year, that the SH hominins are ancestral to Neandertals and that the MtDNA found in later Neandertals is derived relative to that in the SH hominins and that the SH hominins may have had several different MtDNA strands. 

It appears that the evidence supports a model in which the classic Neandertals from France and Germany, which are dated to between 50 and 70 kya, represent a more stabilized, homogenous genome relative to that of earlier hominins.  I have always been struck by the similarities between the Atapuerca 4 cranium and Petralona, from Greece, and this suggests that there may have been extensive migration between regions and considerable gene flow between groups (J. Lawrence Angel, again).  We know that the range of Neandertals extended from the lower Iberian peninsula to Teshik-Tash, north of the Taurus Mountains and Okladnikov, in Siberia.  It does not seem like a stretch that their precursors did the same thing. 

Hat tip to Todd Wood.

New Investigations at the Sima de los Huesos Cave in Atapuerca: Kind of Neandertal and Kind of Not

Science Daily is reporting on a new paper in Science, about the conclusions drawn from the largest single cache of human remains in one place in Europe, the Sima de los Huesos cave, near Atapuerca, in Spain.  They write:

“With the skulls we found," co-author Ignacio Martínez, Professor of Paleontology at the University of Alcalá, added, "it was possible to characterize the cranial morphology of a human population of the European Middle Pleistocene for the first time.”

About 400 to 500 thousand years ago, in the heart of the Pleistocene, archaic humans split off from other groups of that period living in Africa and East Asia, ultimately settling in Eurasia, where they evolved characteristics that would come to define the Neandertal lineage. Several hundred thousand years after that, modern humans -- who had evolved in Africa -- settled in Eurasia, too. They interbred with Neandertals, but even then showed signs of reproductive incompatibility. Because of this, modern humans eventually replaced Neandertals.

This is the hybrid depression that I mentioned a few posts back.  This model fits the genetics information that was summarized by Dennis Venema in his BioLogos post on palaeogenomics from a few weeks ago.  It still also fits with the idea that modern humans swamped a Neandertal genome that was already subject to selective disadvantage.  Consequently, by around 27-29 ky BP, the last populations of Neandertals are, perhaps represented by Zafarraya, a late surviving Neandertal from the very southern coast of Spain. The idea that this Neandertal represents a refugium has been kicked around for decades. Critically, the earliest moderns in Central Europe predate this “last neandertal” by between five and nine thousand years and the evidence from Lagar Velho, in Portugal, indicates that hybridization was occurring as late as this. 

Oldest Human DNA Recovered

Using the fossil material from the Sima de Los Huesos cave, at the Spanish site of Atapuerca, geneticists have recovered DNA.   From the National Geographic story by Karl Gruber:

Analysis of the bones challenges conventional thinking about the geographical spread of our ancient cousins, the early human species called Neanderthals and Denisovans. Until now, these sister families of early humans were thought to have resided in prehistoric Europe and Siberia, respectively. (See also: "The New Age of Exploration.")

But paleontologists write in a new study that the bones of what they thought were European Neanderthals appear genetically closer to the Siberian Denisovans, as shown by maternally inherited "mitochondrial" DNA found in a fossil thighbone uncovered at Spain's Sima de los Huesos cave.

"The fact that they show a mitochondrial genome sequence similar to that of Denisovans is irritating," says Matthias Meyer of Germany's Max Planck Institute for Evolutionary Anthropology in Leipzig, lead author of the study, published Wednesday in Nature.

"Our results suggest that the evolutionary history of Neanderthals and Denisovans may be very complicated and possibly involved mixing between different archaic human groups," he said.

Although the report argues that the Atapuerca remains show a greater similarity to Denisovans than Neandertals, we already know that modern Europeans and East Asians have Neandertal genes in them, indicating that admixture was occurring.  Milford Wolpoff has been arguing for decades that archaic Homo sapiens represents a polytypic species that has genetic ties to modern human groups in different parts of the Old World.  While it certainly appears that Neandertals were distinctive, this information, in conjunction with various studies (here and here) of the Denisovans and African archaics indicates that this model may be the more correct one. 

Lost in the shuffle, however, is that the ability to recover this DNA is an astounding feat of science and portends for great advances in human palaeontology in the future. 

Fossil Data and Genetic Data More Concurrent Than Thought

It is now coming out that the “clocks” that are used to calculate mutation rates are badly calibrated. Ewen Callaway of Nature writes:

Geneticists have previously estimated mutation rates by comparing the human genome with the sequences of other primates. On the basis of species-divergence dates gleaned—ironically—from fossil evidence, they concluded that in human DNA, each letter mutates once every billion years. “It’s a suspiciously round number,” says Linda Vigilant, a molecular anthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. The suspicion turned out to be justified.

Now it seems that the clock is off by half. This means that a bunch of divergence dates can be recalculated. One is particular is mentioned:

Take the 400,000–600,000-year-old Sima de Los Huesos site in Atapuerca, Spain, which yielded bones attributed to Homo heidelbergensis, the direct ancestors of Neanderthals. Genetic studies have suggested that earlier ancestors of Neanderthals split from the branch leading to modern humans much more recently, just 270,000–435,000 years ago. A slowed molecular clock pushes this back to a more comfortable 600,000 years ago.

It isn't a magic bullet, there are still some kinks to be worked out but it has remarkable potential for getting a handle on when the Denisova genome might have arisen.It might also give us a believable figure for the LCA of apes and humans since the current, 5 million year date is plainly not so.

Middle Pleistocene Hominin Height

Science Daily has a story on work being done with the material from the Sima de los Huesos cave at Atapuerca. 27 complete long bones have been discovered there and from these, it has been possible to estimate the height of Homo heidelbergensis. Science Daily writes:

The results suggest that both men and women in the Sima de los Huesos population were on average slightly higher than Neanderthal men and women. "Neither can be described as being short and both are placed in the medium and above-medium height categories. But, both species featured tall individuals," assured the experts.

The height of these two species is similar to that of modern day population of mid-latitudes, like in the case of Central Europe and the Mediterranean.

The humans who arrived in Europe during the Upper Palaeolithic era, Cro-Magnons or anatomically modern humans, replaced the Neanderthal populations. They were significantly taller than other human species and their average height for both sexes was higher, falling in the very tall individual category.

Height remained the same for some 2 million years.

We know from the remains at Turkana that, at least for some individuals, height close to that of modern humans had been achieved by 1.3 million years ago.This probably reflects (amongst other things) a massive increase in protein in the diet, which would come about by increased hunting, something present in late African Homo ergaster. More pieces of the puzzle.

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“Oh, My Aching Back!”

Researchers working with human fossil material between 400 and 500 thousand years ago, from the Sima de Los Huesos, have discovered a Homo heidelbergensis individual that had some serious back issues. Susan Uzel of Cambridge News reports:

Found among the bones of around 28 people at a site called Sima de los Huesos – “pit of bones” – in northern Spain, the lumbar spine was reconstructed from fragments discovered by scientists from the Centro Mixto de Evolución Humana in Burgos.

The lumbar spine comes from the same man as a pelvis found in 1994. It is thought he was aged 45, and lived more than half a million years ago.

The way in which the bones developed and the way they changed due to wear and tear show he was likely to have suffered severe back pain.

Dr Gomez-Olivencia said: “It appears that we are looking at the spine of a man who had several different problems, including inversion of the curvature of the back, spondylolisthesis, and Baastrup disease – which are associated with pain today.”

Homo heidelbergensis were nomadic hunter gatherers, relying on animals such as red deer for food, and a damaged spine would have made hunting impossible.

This is not so unusual. With bipedalism comes all sorts of issues with the back. We put an incredible amount of strain on the lower vertebrae, which often causes herniation of the discs. As we get larger and heavier (read: fatter), these get exacerbated.

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