Monday, October 31, 2016

How Long Ago Did Homo Naledi Roam Southern Africa?

H. naledi is the most recent branch of the genus Homo that has been discovered, with remains collected in a cave crypt in South Africa. But, neither ancient DNA nor a reliable radioactive method date of the samples has been possible to date, so scientists have had to resort to old fashioned guesstimating based upon observed features of the H. naledi remains to estimate their age. 

In any case, any time estimate would only establish a point in time at which the species existed at that location, because there is only a single site that appears to involve individuals who died at roughly the same time, plus or minus a few centuries.

For example, despite the evidence that Khoi-san hunter-gatherers who now live nearby had archaic admixture of a type different than Neanderthals and Denisovans from an event definitely in the last 0.1 million years and probably in the last 0.01 million years, the 2.5 million to 0.9 million time depth of H. naledi makes this species a pretty poor candidate for the source of that admixture, although the admixture could have involved a species descended from H. naledi. But, the crypt could have been created at any time in a range from shortly before H. naledi became extinct, to shortly before it evolved as a new species.

There is at least one inaccuracy in the story below. Homo erectus dates to more like 2.0-2.1 million years ago in Africa, rather than 1.8 million years as stated. There are examples of H. erectus in Asia from 1.9 million years ago and the species almost certainly evolved first in Africa somewhat before that date.
Evolutionary trees of ancient hominids statistically reconstructed from skull and tooth measurements indicate that H. naledi lived around 912,000 years ago, say paleoanthropologist Mana Dembo of Simon Fraser University in Burnaby, Canada, and her colleagues. That’s a provisional estimate, since researchers have yet to date either H. naledi’s bones or the sediment in which some of its remains were excavated. 
The new statistical age estimate, described by Dembo’s group in the August Journal of Human Evolution, challenges proposals that H. naledi’s remains come from early in Homo evolution. Researchers who first studied H. naledi bones retrieved from an underground cave in South Africa noted similarities of the skull and several other body parts to early Homo species dating to between 2.5 million and 1.5 million years ago (SN: 10/3/15, p. 6). 
A comparison of H. naledi skull measurements to those of 10 other hominid species, conducted by paleoanthropologist J. Francis Thackeray of the University of the Witwatersrand in Johannesburg, reached the same conclusion. H. naledi lived roughly 2 million years ago, Thackeray proposed in the November/December 2015 South African Journal of Science. 
Dembo disagrees. Her team tested which of 60,000 possible evolutionary trees best fit skull and tooth measurements of H. naledi, 20 other hominid species, gorillas and chimpanzees. The new analysis keeps H. naledi in the genus Homo. But it’s still unclear which of several hominid species — including Homo sapiens, Homo floresiensis (or “hobbits”) and Australopithecus sediba (SN: 8/10/13, p. 26) — is most closely related to the South African species. 
Dembo’s team found no signs that bones assigned to H. naledi represent a variant of Homo erectus, as some scientists have argued. H. erectus originated about 1.8 million years ago in Africa and rapidly spread to West Asia. But Dembo’s statistical model assumes that H. erectus skulls and teeth vary in shape throughout Africa and Asia much less than they actually do, says paleoanthropologist Christoph Zollikofer of the University of Zurich. Bones assigned to H. naledi most likely represent a form of H. erectus, he argues. 
Further statistical comparisons that include measurements of limb and trunk bones may help to clarify H. naledi’s evolutionary relationships, Dembo says. 
Based on geological dates for all hominids except H. naledi, the researchers also calculated the rate at which each species’ skull and tooth features evolved over time. Those results enabled the researchers to estimate H. naledi’s age. 
Homo naledi might be less than a million years old,” Dembo says. She considers that estimate “reasonably robust,” since ages calculated for other hominids in the analysis often fell close to dates gleaned from fossil and sediment studies. In a few cases, though, statistical and geological age estimates differed by 800,000 years or more.
From Science News.

John Hawks has some interesting recent H. naledi remarks, mostly in response to Thackeray's paper, in connection with the published response from his team cited below.

The Science News story cites the following papers:

* M. Dembo et al. The evolutionary relationships and age of Homo naledi: an assessment using dated Bayesian phylogenetic methods. Journal of Human Evolution. Vol. 97, August 2016, p. 17. doi: 10.1016/j.jhevol.2016.04.008.

* J. Hawks and L. Berger. The impact of a date for understanding the importance of Homo naledi. Transactions of the Royal Society of South Africa. Vol. 71, issue 2, 2016. doi: 10.1080/0035919X.2016.1178186.

* J.F. Thackeray. Estimating the age and affinities of Homo naledi. South African Journal of Science. Vol. 111, November/December 2015, p. 3. doi: 10.17159/sajs.2015/a0124.

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