Monday, October 5, 2015

Homo nadeli and the Problems with the Homo Genus

Except for those living in a cave (like the one that Homo naledi was found in), the whole world has been abuzz with news of the discovery of an entirely new species of human "ancestor." I place "ancestor" in quotes because much work remains before we'll know whether our species is a direct descendent of H. naledi or if they are merely long-lost cousins like Neanderthal and Homo erectus.

So much about this story is incredible. First of all, the find itself was unprecedented. Never have so many hominin fossils been found all at once. Literally thousands of bone fragments were found, most of them largely intact, allowing the recreation of nearly the entire skeleton. At least 15 individuals are present among the remains. For comparison, we know of Australopithecus afarensis (the species of the famous "Lucy") from just seven individuals and a much more incomplete skeleton. There are other putative species that are represented by a single jaw bone or skull cap. From a single site, we have more fossil evidence of Homo naledi than we do for all but a few hominin species.




Homo naledi skeleton
From Berger et al. (2015)
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Secondly, the working conditions of the recovery team were unbelievably demanding. While fossil digs often take place in harsh conditions, especially in Africa, the Rising Star cave was like no other. The subterranean chamber that held the fossil treasure is accessible only through a labyrinth of shafts and passageways. To reach it, the diggers had to rappel down narrow shafts, crawl through tiny crevices, and surmount a huge hilly boulder. After more than 200m of spelunking, the Dinaledi chamber is at the very terminus of the cavern, a mere 10x3 meter hollow.

So tight were the squeezes that anthropologist Lee Burger specifically sought "skinny and small" people for his team, as seen on the original Facebook post, which still exists.

From 20 applicants, he selected six slender women. Good thing, too. One of the crevices blocking the bath is so narrow that the fossil hunters had to squeeze through with their heads turned to the side, while crawling on their belly. Oh, and remember that the cave is in complete darkness. This was truly a Herculean effort.

In the tiny chamber at the end of the long, musty, dark, subterraneous journey lay one of the biggest treasures in the history of anthropology. Scattered on the floor were the bones of many individuals, most not buried at all. What were they doing there? We'll probably never know. Speculation has already begun that this was an ancient burial site because the cave is extremely old and would have been just as inaccessible and just as dark in the distant past as it is now. However, there are a number of plausible explanations, and many anthropologists are quite incredulous at the notion that these hominins engaged in deliberate burial behavior. I suspect this will be the topic of speculation for many years.

The bones have not been definitively dated, but early estimates age them at around 2.5 million years. That is part of what makes the find so exciting for paleoanthropologists. Fossils in that age range are scarce, leaving us with quite a "black box" regarding the origins of our lineage from the variety of more primitive hominins that were roaming through Africa at that time.

The informal taxon known as Australopithecines, or simply Australopiths, includes three or four genera and 10–13 different species. These not-quite-human-beings experienced an impressive array of adaptive radiation as they experimented with the various ecological niches in the lush African rainforests and savannahs of the Pliocene and early Pleistocene epochs. Each species is an interesting mix of more primitive "ape-like" features and more advanced "human-like" features. One of these species gave rise to the Homo genus and was the ancestor of habilis, erectus, Neanderthal, and modern humans. We don't yet know which one.

Prior to the discovery of H. naledi, the oldest known member of Homo was habilis, which is, or was, believed to be the foundational species of the Homo genus and the ancestor of all the Homo hominins that came later. The oldest fossils that unquestionably belong to habilis date to around 1.9 million years ago.

Preliminary estimates place the Homo naledi remains at around 2.5 million years of age. This means that naledi might be much older than habilis, replacing her as the foundational species of the Homo genus.

But there's a problem. While the placement of naledi within the Homo genus by Lee Burger and his team is in keeping with current practice in paleoanthropology, it is not without controversy. In fact, Homo naledi could be the bones that break the camel's back regarding a simmering problem in the field: the Homo genus has never been properly defined.

Scientist in Rising Star Cave
From National Geographic: Sunlight falls through the entrance of Rising Star cave, near Johannesburg. A remote chamber has yielded hundreds of fossil bones—so far. Says anthropologist Marina Elliott, seated, "We have literally just scratched the surface."

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The practice of taxonomy is usually governed by a rigid process involving the identification of derived characteristics that are used to strictly define a taxon. While revisions are not uncommon if and when a definition becomes untenable in the face of new discoveries, such upheaval is carefully managed by the scientific consensus. There is one exception to this orderly classification process: paleoanthropology.

Since the early human fossil record began to slowly emerge from the earth, the boundaries and definitions of the Homo genus have been poorly defined and constantly in flux. Because there was no reason to think it would ever have any additional members, Carl Linnaeus did not even bother to define the Homo genus when he first created it for humans in the 1700s. The discovery of Neanderthal brought the first addition. This was a fairly straightforward decision given the striking anatomical similarities to Homo sapiens, which has been further validated by the DNA evidence showing interbreeding between humans and Neanderthals. Subsequent to Neanderthal, many further hominins have been lumped in with Homo based on ever-changing criteria.

Ian Tattersall and Jeffrey Schwartz wrote in Science just this past August that "the boundaries of both the species and the genus remain as fuzzy as ever, new fossils having been rather haphazardly assigned to species of Homo." Various anatomical definitions of Homo have been proposed, usually involving the bones of the face, but also inclusive of posture. But these proposed boundaries have given way to the continued lumping of newly discovered species. Louis Leakey, one of the key founders of paleoanthropology, thought of Homo as the tool-making genus, but this is hardly practical given that behaviors don't fossilize so well.

The controversy over Homo is perhaps best symbolized by the fact that the supposed foundational species, habilis, is itself not unquestionably a member of Homo. Many prominent anthropologists, including Richard Leakey and Bernard Wood, have argued that habilis is an Australopith and prefer the name Australopithecus habilis. As Wood wrote in Nature last year, "…if H. habilis is added to Homo, the genus has an incoherent mishmash of features."

It is in this muddled context that Homo naledi has burst onto the scene. As Homo is the genus that means "man", H. naledi may end up snatching the mantle of "the first humans" away from habilis. However, this only magnifies the problems. naledi, like other early hominins, is a mosaic of primitive and derived features.

The hands of H. naledi appear adapted for high-level fine-motor skills, yet harbor long, curved fingers, indicating an at least partially arboreal lifestyle. She walked fully upright, but was shorter than most members of Homo. The bones of her upper body were more similar to Australopiths than to Homo. Her cranial capacity was also quite small, around 500cc. (Compare this to that of habilis, ergaster, and erectus at >600cc, 700–900cc, and as much as 1000cc, respectively.) Including naledi in Homo brings all of the problems of the genus into focus.

Hominin skulls
From Stringer (2015).
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In any other genus, there would be clear inclusion/exclusion criteria and one or two of the features above would be considered the derived characteristic(s). If you have it, you're in; if you don't, you're not. This process begins at the top-level taxon and works its way down. For example, the lack of a tail places a species in the family Hominidae, but only if it has already been included in the order primata. Once we arrive at the Homo genus, however, many criteria are considered collectively and the result is a tendency of extreme lumping together, when multiple genera are probably called for.

The discovery of naledi, given how much more primitive it is than habilis, might be the event that finally forces the paleoanthropology community to get serious about its taxonomy. Don't be surprised if many of our ancestors get new names in the coming years.

Homo naledi might be the most spectacular fossil find of this decade. But her genus is a mess.
Nathan H. Lents, Ph.D. is a Professor of Molecular Biology at John Jay College of The City University of New York and the author of the upcoming book, "Not So Different: Finding Human Nature in Animals" (Columbia University Press, 2016). He maintains The Human Evolution Blog and is also a contributor for clapway.com. You can follow him on Twitter, Facebook, and Tumblr.

Works Cited

  • Berger, L. R., Hawks, J., de Ruiter, D. J., Churchill, S. E., Schmid, P., Delezene, L. K., … & Zipfel, B. (2015). "Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa." eLife, 4, e09560.
  • Green, R. E., Krause, J., Ptak, S. E., Briggs, A. W., Ronan, M. T., Simons, J. F., … & Pääbo, S. (2006). "Analysis of one million base pairs of Neanderthal DNA." Nature, 444(7117), 330–336.
  • Howell, F. C. (1951). "The place of Neanderthal man in human evolution." American Journal of Physical Anthropology, 9(4), 379–416.
  • Linnaeus, C. (1788). Systema naturae per regna tria naturae secundum classes, ordines, genera, species. (Vol. 1). impensis Georg Emanuel Beer.
  • Schwartz JH and Tattersall I. "Defining the genus Homo." Science. 28 August 2015. v349:6251. pp931–932.
  • Stringer, C. (2015). "The many mysteries of Homo naledi." eLife, 4, e10627.
  • Wood, BJ. "Human evolution: Fifty years after Homo habilis." Nature. 03 April 2014. v508. pp31–33.

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