Did Humans Become a Distinct Species 350, 000 Years Ago?

The hits just keep on coming.  On the heels of the discovery in Morocco, at Jebel Irhoud, of a skull with some modern characteristics dated to 315 thousand years ago, we now have a genetic study that proposes that our species emerged between 350 and 260 thousand years ago.  How did a genetic study reveal this?  Science News reports:

The trick was retrieving a complete version of the ancient boy’s DNA from his skeleton to compare with DNA from people today and from Stone Age Neandertals and Denisovans. Previously documented migrations of West African farmers to East Africa around 2,000 years ago, and then to southern Africa around 1,500 years ago, reshaped Africans’ genetics — and obscured ancient ancestry patterns — more than has been known, the researchers report online September 28 in Science.

The ancient boy’s DNA was not affected by those migrations. As a result, it provides the best benchmark so far for gauging when Homo sapiens originated in Africa, evolutionary geneticist Carina Schlebusch of Uppsala University in Sweden and her colleagues conclude.

In line with the new genetically derived age estimate for human origins, another team has proposed that approximately 300,000-year-old fossils found in northwestern Africa belonged to H. sapiens (SN: 7/8/17, p. 6). Some researchers suspect a skull from South Africa’s Florisbad site, dated to around 260,000 years ago, qualifies as H. sapiens. But investigators often place our species’ origins close to 200,000 years ago (SN: 2/26/05, p. 141). There is broad consensus that several fossils from that time represent H. sapiens.

What is important to understand is that we have the fossils from Herto and Omo that date to between 150 and 200 thousand but that does not mean that is the earliest time that our species may have actually showed up. The Jebel Irhoud material is only partly modern—in the face mostly—and we do not know how the genetics for this population would look. It is possible that, as some investigators are saying, the Jebel Irhoud material represents the earliest fossil representation of our clade and that over the next 100 thousand years, the modern form fully evolved.   

Surprising New Information About Homo naledi

The Atlantic (and other outlets) has a story on new information on the date of the Homo naledi fossils from Rising Star Cave and it is...surprising.  As Sarah Zhang writes:

The one thing everyone agrees is that the fossils themselves are spectacular. In 2015, researchers unveiled 1,500 hominin fossils found deep in a South African cave, excavated by six cavers who were all skinny, short, and female. The hominin, a new species the team christened Homo naledi, was an unusual mix of the old and modern. Their heads were small, suggesting an early hominin perhaps more than a million years old. But their feet were stiff for walking upright and their hands adept like modern humans.

So in the media frenzy that followed—a National Geographic cover, a documentary, numerous articles—the question kept coming up: How old are these Homo naledi fossils, really? What do they tell us, if anything, about the origin of Homo sapiens?

To that first question, the researchers now have an answer: 236,000 to 335,000 years old. As for the second question, well, it’s complicated. “You can’t tell simple stories anymore,” says Lee Berger, a paleoanthropologist at the University of the Witwatersrand who led the research. “This is the gigantic message out of Homo naledi.” The age of these fossils puts these strange, small-brained yet human-like hominins in South Africa just before the emergence of the first anatomically modern Homo sapiens.

Holy Liang Bua, Batman!  If the bones really are 236k, then that would put them roughly 40-50 k years before the advent of modernity represented by the Omo 1 cranium.  Admittedly, that skull is fragments “swimming in plaster” but we have a pretty good idea of what it looks like. We certainly have modernity in the Bouri remains from Herto at 160 ky.

How do we know how old the fossil are?  From the eLife article:

Now, Dirks et al. – who include many of the researchers who were involved in the discovery of H. naledi – report that the fossils are most likely between 236,000 and 335,000 years old. These dates are based on measuring the concentration of radioactive elements, and the damage caused by these elements (which accumulates over time), in three fossilized teeth, plus surrounding rock and sediments from the cave chamber. Importantly, the most crucial tests were carried out at independent laboratories around the world, and the scientists conducted the tests without knowing the results of the other laboratories. Dirks et al. took these extra steps to make sure that the results obtained were reproducible and unbiased.

More specifically, they did Electron Spin Resonance (or Electron Paramagnetic Resonance), Uranium-Thorium disequilibrium, Optically Stimulated Resonance and radiocarbon dating.  This is probably not the last word on the dating of the fossils and there are quite a few assumptions that have been made regarding the provenance of the fossils to the cave sediments, but this is the closest thing that we currently have. One of the critical problems in dating much of the South African cave deposits is their relationship to the hominins found there.  Often, the caves opened up at the top and the fossil remains either fell in, were dropped in or they crawled in.  What makes the Rising Star remains so peculiar is that this cave did not form in the traditional way and the bones were so far into the cave. 

Where does this leave us? It leaves us with more questions than answers.  It has been common consensus that, while there was considerable variability in hominin morphology throughout the Pliocene and early to middle Pleistocene, that this "settled down" and that variability diminished as we made the transition to modern humans.  These fossils suggest the opposite: that large amounts of variability were, in fact, present throughout the range of human existence and only recently (200ky to the present) did this variability drop off.

I am quite sure that more studies will be done to elucidate the relationship that these fossils (there are more very similar fossils next door in the Lesedi cave complex) have to other forms in Africa.  There are obviously very peculiar cultural interactions going on, though (or lack, thereof).  With the Liang Bua remains in East Asia, hypotheses of endemic dwarfism and isolation have been put forth to explain why these remains are so diminutive and primitive.  It is more difficult to put forth these same arguments here where there are few to no barriers to migration and interaction between groups. 

Study: Neandertal Speech and Language Abilities Like Modern Humans

Science Daily has a story on Neandertal speech.  Work by Dan Didiu and Stephen Levinson suggests that Neandertals and modern humans share a common ancestry with regard to the use of language.  Science Daily writes:

Dediu and Levinson review all these strands of literature and argue that essentially modern language and speech are an ancient feature of our lineage dating back at least to the most recent ancestor we shared with the Neandertals and the Denisovans (another form of humanity known mostly from their genome). Their interpretation of the intrinsically ambiguous and scant evidence goes against the scenario usually assumed by most language scientists, namely that of a sudden and recent emergence of modernity, presumably due to a single -- or very few -- genetic mutations. This pushes back the origins of modern language by a factor of 10 from the often-cited 50 or so thousand years, to around a million years ago -- somewhere between the origins of our genus, Homo, some 1.8 million years ago, and the emergence of Homo heidelbergensis. This reassessment of the evidence goes against a saltationist scenario where a single catastrophic mutation in a single individual would suddenly give rise to language, and suggests that a gradual accumulation of biological and cultural innovations is much more plausible..

I am still a bit “iffy” on the whole “common ancestor of modern humans and Neandertals” thing since some are trying to argue for a common ancestor based on the material from the Gran Dolina, in Spain, and it sure as all get out looks like modern humans originated in Northeast Africa at, or near, Herto. I have a tendency to think that there was a good deal of panmixia throughout Europe and north Africa.  Otherwise, we are required to believe that there was a species split at H. antecessor into what became modern humans (how'd they get to north Africa?) on one side and Neandertals on the other and that, some 700 to 720 thousand years later, emerging moderns and Neandertals encountered one another and interbred.  That is overly simplistic, I am sure but it seems far-fetched.  It seems easier to argue that the H. antecessor material represents an archaic Homo sapiens population that was the precursor (or something like it was) to later archaics, such as Petralona and Atapuerca, out of which Neandertals came.  Then the Neandertals interbred with the north-moving moderns and somewhere in there, the Denisova population split off.  The panmixia in Europe, western Asia and Russia explains why moderns have both Neandertal and Denisovan genes, along with the modern genes that originated in North Africa.  Subsequent swamping of the archaic genome led to the demise of both the Neandertals and Denisovans.   

More Trouble for the Complete "Out-of-Africa" Model

Science Daily is reporting on research done on migration patterns involving early anatomically modern humans.  They write:

The team, led by Johannes Krause from Tübingen University, was able to reconstruct more than ten mitochondrial genomes (mtDNAs) from modern humans from Eurasia that span 40,000 years of prehistory. The samples include some of the oldest modern human fossils from Europe such as the triple burial from Dolni Vestonice in the Czech Republic, as well as the oldest modern human skeletons found in Germany from the site of Oberkassel close to Bonn.

The researchers show that pre-ice age hunter-gatherers from Europe carry mtDNA that is related to that seen in post-ice age modern humans such as the Oberkassel fossils. This suggests that there was population continuity throughout the last major glaciation event in Europe around 20,000 years ago. Two of the Dolni Vestonice hunter-gatherers also carry identical mtDNAs, suggesting a close maternal relationship among these individuals who were buried together.

It was also suggested that the split between non-Africans and Africans was much later in time, between 62 and 95 ky BP. This would mean that the window of hybridization would be much larger than originally thought and that archaic and modern hybridization was a good deal more common until a later time than thought.  The first moderns that we have are from the site of Bouri, in the Afar Triangle, and date to around 160 ky BP.  Maddeningly, what we are lacking is good material between that time and around 100 ky BP, when the Near Eastern Skhul and Qafzeh material are found.  A friend of mine and I argued that the archaic traits in those skulls represent African archaic traits and not hybridization with Neandertals.  This would seem to support that.  They suggest that there are discrepancies between these results and previous ones but, critically, that these results tend to support both the palaeoanthropological and archaeological evidence.    Here is the citation:

Fu et al., A Revised Timescale for Human Evolution Based on Ancient Mitochondrial Genomes,
Current Biology (2013), http://dx.doi.org/10.1016/j.cub.2013.02.044

Modern Humans Out of Africa Earlier Than Thought?

The Australian is reporting on the find in Arabia of stone tools purported to be crafted to modern humans that date to around 100 000 years ago (date derived by thermoluminescence). They write:

Jeffrey Rose, an archaeologist with Britain's Birmingham University and head of the Dhofar Archaeological Project, said: "After a decade of searching in southern Arabia for some clue that might help us understand early human expansion, at long last we've found the smoking gun of their exit from Africa."

The discovery also challenges current thinking about the route the migrants took, say Dr Rose and his colleagues, including geochronologist Bert Roberts of Wollongong University.

The article shows up in PLoS¹. This is intriguing and certainly adds one more piece of the puzzle but I am not sure why it so earth-shattering. We already know that we have hominins in the Levant that pass, in most ways, for modern humans and are dated to between 90 and 110 000 B.P. We also have the Herto remains at 140-160 000, which are also mostly modern. A glance at the map shows that the gap between the Horn of Africa and the Arabian peninsula is very narrow here and it would not have been a problem to navigate the waters at this point. This just represents another migration route of many. The authors of the paper call the assemblage that was discovered a Nubian variant of the “Middle Stone Age,” which tracks as a post-Acheulean industry and is present in various forms in Africa from about 300 000 years B.P. down to around 40 000 or so. Here is the map from the article:



As you can see, there is Nubian in both Arabia and in the Nile valley. Interestingly, when you get to the Levantine peninsula, you run into “modern” humans creating Levantine Mousterian, another Middle Palaeolithic industry, which means they had contact with the Neandertals who were escaping the cold to come south during the early Würm glaciation. It is during the Early Würm/Late Würm interglacial period that the early moderns get the bright idea to head north, out of the Levant.

¹Rose JI, Usik VI, Marks AE, Hilbert YH, Galletti CS, et al. (2011) The Nubian Complex of Dhofar, Oman: An African Middle Stone Age Industry in Southern Arabia. PLoS ONE 6(11): e28239. doi:http://dx.doi.org/10.1371/journal.pone.0028239


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