Neandertal to Modern Transition Full of Fits and Spurts

New evidence suggests that Neandertals survived longer on the Iberian peninsula than elsewhere in Europe.  The redating of the Zafarraya Neandertal from the late 20k's to between 38 and 40 ky saw a revision of theories about Neandertal extinction. Now more evidence has been unearthed suggesting that this window is more secure.   From The Statesman:

Neanderthals may have survived at least 3,000 years longer than thought in what is now Spain – much after the species had died out everywhere else, a study has found.

The findings suggest that the process of modern human populations absorbing Neanderthal populations through interbreeding was not a regular, gradual wave-of-advance but a “stop-and-go, punctuated, geographically uneven history.”

Over more than ten years of fieldwork, researchers excavated three new sites in southern Spain, where they discovered evidence of distinctly Neanderthal materials dating until 37,000 years ago.

“Technology from the Middle Paleolithic in Europe is exclusively associated with the Neanderthals,” said Joao Zilhao, from the University of Barcelona in Spain.

“In three new excavation sites, we found Neanderthal artefacts dated to thousands of years later than anywhere else in Western Europe,” said Zilhao, lead author of the study published in the journal Heliyon.

It seems pretty clear now that the model of rapid replacement of Neandertals does not hold. This kind of evidence, along with sites such as Lagar Velho, suggest that the transition, at least in this region was long-term, with much hybridization.  We still do not know exactly how long Neandertals persisted in this region.  It makes perfect sense that they would have been the last ones to undergo the transition (however that happened) because the earliest Aurignacian sites, associated with modern humans, occur in far eastern Europe as the wave of moderns came through during the Würm I/Würm II interglacial.

The question that I have is why didn't the earliest modern humans, represented by the Jebel Irhoud remains, come through the Strait of Gibraltar?  North of that, all you find are archaic, all the way down to the Zafarraya remains.  I wonder why this is.

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. 

Dennis Venema on Human Evolution and Palaeogenomics

Dennis Venema is doing the genetics end of human evolution.  He has, as with most evolutionary creationists, taken the long view of creation, to the point where he has publicly stated that the modern human genome could not possibly have originated with two individuals.  Knowing what I do of the fossil record (upcoming BioLogos posts on June 16-18), I have no problem with that.  He focuses on palaeogenomics, and writes:

The discovery of Neanderthal remains in the same location that preserved the exceptional Denisovan DNA at last provided a very high quality Neanderthal genome, the analysis of which was published just this year. This new data allowed for several more robust analyses comparing human, Denisovan, and Neanderthal genomes. These analyses produced several noteworthy results: modern mainland Asian populations also have a small amount of Denisovan DNA; humans contributed some DNA variation to Neanderthals; Neanderthals and Denisovans interbred, with DNA variation flowing both ways; and (perhaps most interestingly) Denisovans have DNA variation that suggests they interbred with yet another archaic hominin group, possibly Homo erectus.

While the palaeogenomic data indicate a split time of c. 600 Ky years for modern humans and Neandertals, it is fairly clear from other genetic evidence that there was at least some interbreeding between early moderns and Neandertals all the way up to around 50 Ky B.P. (perhaps as recently as 29 Ky, given the information from the Portuguese burial at Lagar Velho), as well as interbreeding between the newly emerging African moderns and African archaic Homo sapiens. 

The news that the Denisovans have some very archaic DNA in them is not surprising.  IF modern humans and Neandertals split around 600 Ky B.P., the only people running around on the landscape at that point were early archaic Homo sapiens/late Homo erectus.  The salient feature of the name "archaic Homo sapiens" is that it is not a taxonomic name but rather a linguistic "catch-all."  The primary reason for this is because, as I noted in my series on human origins, the earliest members of this group still have some Homo erectus characteristics, while the later ones look more modern.  When these fossils, which all date to between (possibly ) 800 and 400 Ky B.P., were pulled out of the ground, researchers did not know what to call them, so they applied this "catch-all."  Adding to the confusion is that some of the archaic Homo sapiens crania from Africa bear more than a passing similarity to some of the finds in Europe.  All were clearly transitional but what to call them was a mystery.  Researchers such as Bernard Wood, applying systematics to the problem, have suggested that these forms should be given the name Homo heidelbergensis.  Such a taxon would, however, have quite a lengthy geomorphochronocline, stretching its limits, perhaps, to the breaking point. For a refresher on these fossils, you might want to read this post. 

So, if the split between modern humans and Neandertals was around 600 Ky B.P., who would have been the progenitors of these groups?  Modern humans don't appear on the landscape until between 160 and 190Ky B.P.  Neandertals don't appear until maybe 300 Ky B.P.  Here is one possible scenario. Sometime prior to 600k B.P., there was a migration out of Africa by one or more groups of archaic Homo sapiens/late Homo erectus.  This explains the similarities between the Petralona and African Kabwe crania, as well as that of the Gran Dolina remains and the African Bodo cranium.  In Africa, the archaics went about their business being archaic until around 200 ky B.P., when the modern genome began to arise (in response to what?).  In Europe, over time, in the face of not one but two ice ages (The Riss and Würm), one group evolved into the Neandertals, who's remains are found in Europe, the Levant, northern Iraq and very western Russia. Populations in the two areas would have, depending on the level of gene flow in the circum-Mediterranean area, become genetically isolated.

Now here is the really odd thing.  When these groups reunited, so to speak, they discovered that, not only was there an attraction, they were still genetically compatible after some 500 thousand years apart—to a point. Even though they actually produced viable offspring (and the presence of Neandertal genes in modern humans demonstrates that they were), because the Neandertal and modern human genomes were "optimized" if you will, eventually over time, hybrid depression would likely have ensued, reducing the fitness of the hybrid offspring.  This may account for the ultimate demise of the Neandertals, who had a genome that, with the warming at the end of the early Würm stadial, was likely undergoing negative selection pressure.  If, in fact, there was a swamping of the Neandertal genome by arriving moderns through the gates of Europe, this would have sped up the process.

This scenario is bare bones at best but explains much of the fossil data.  It leaves aside the hotly contested notion of whether or not Neandertals were a separate species from the early moderns and, with them, reflected a syngameon, or whether, as Milford Wolpoff contends, they reflect a widely polytypic species that interbred at the peripheries.  That is for another day. 

Neandertal, Early Modern Hybridization

The Telegraph has a story about research indicating a shared history between Neandertals and early modern humans. This story appeared a few days ago in the New York Times. I did not link to this. Richard Alleyne, the Telegraph science correspondent writes:

Human-Neanderthal relations occurred as the first pioneering bands of homo sapiens ventured out of Africa, scientists believe.

When they reached the Middle East they ran into groups of Neanderthals who preceded them and it is now thought that they mated.

The discovery emerged from the first attempt to map the complete Neanderthal genetic code, or genome. It more or less settles a long-standing academic debate over interbreeding between separate branches of the human family tree.

Evidence in the past has pointed both ways, for and against modern humans and Neanderthals mixing their genes.

This is support for the contention of Erik Trinkaus that the Lagar Velho burial in Portugal reflects hybridization, as well as the findings of Dave Frayer and Fred Smith, who have long argued that the early modern humans in Europe from Mladec, Stetten, Predmosti and several other sites have traits present in their Neandertal antecedents from the area. This will certainly add fuel to the fire in the continuity/replacement debate concerning the appearance of modern humans.

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No, It Is Probably Not a "Chunky Gravettian Kid."

Research on the teeth of the Lagar Velho specimen has revealed evidence suggesting that Zilhao and Trinkaus were right in their initial assessment of the evolutionary placement of the specimen. Science Daily reports:

Employing a technique called micro-tomography which uses x-rays to create cross-sections of 3D-objects, the researchers investigated the relative stages of formation of the developing teeth and the proportions of crown enamel, dentin and pulp in the teeth.

They found that, for a given stage of development of the cheek teeth, the front teeth were relatively delayed in their degree of formation. Moreover, the front teeth had a greater volume of dentin and pulp but proportionally less enamel than the teeth of recent humans.

The teeth of the Lagar Velho child thus fit the pattern evident in the preceding Neanderthals, and contrast with the teeth of later Pleistocene (12,000-year-old) humans and living modern humans.

Expect another dust-up.

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The Last Neandertals and Climate Change

Joao Zilhao is now suggesting that the last surviving Neandertals had to have died out around 37 ky, rather than 27 ky which is what has been thought for the last several years. According to a story in Yahoo News:

The paper, by Professor Zilhao and colleagues from the University of Bristol, revealed new dating evidence for the Late Aurignacian of Portugal, an archaeological culture associated with modern humans, which firmly constrains the age of the last Neanderthals of southern and western Iberia to some 37,000 years ago.

This new evidence puts the duration of the Iberian Neanderthal refugium at five millennia, and counters speculations that Neanderthal populations could have remained in the Gibraltar area until 28,000 years ago.

These findings have important implications for the understanding of the archaic features found in the anatomy of a 30,000-year-old child unearthed at Lagar Velho, Portugal.

This will be hard to square with Zafarraya as well. The stone tool assemblages from that site are clearly Mousterian with, as Jean-Jacques Hublin writes: "little or no Upper Palaeolithic influence." I also think that Erik Trinkaus' analysis of the Lagar Velho remains, is very close to the mark. I simply do not think that it is credible to view the remains as that of a "chunky Gravettian kid."

I wonder if this will revive the incredible dust-up that followed, regarding the analysis of the Lagar Velho remains. I challenge anyone out there to find Erik's original post. It seems to have vanished into the ether.

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