Human Origins

Recently, I came across a debate in an NPR comments forum about the origins of modern humans in the African continent.  Predictably, the debate got a bit heated.  For what it's worth, here is a summary of (some) key data in the paleontological, archaeological, and molecular genetics records.

I.) Ian Tattersall (2009) reviews the literature in a relatively short and quite readable article in the journal Proceedings of the National Academy of Sciences of the United States (PNAS).  Key points:

i.) In his section on Fossil Record of H. Sapiens, Tattersall writes:

How and where, then, were the distinctive features of H. sapiens acquired? The relevant fossil record is fairly thin, but it does firmly establish that as an anatomically recognizable entity our species made its first appearance in Africa (23, 24). The “Out of Africa” hypothesis of modern human origins emerged in the mid-1980s, when paleoanthropologists such as Günter Bräuer in Germany (e.g., ref. 25) and Chris Stringer in the U.K. (e.g., ref. 26) began to point out that, sparse as they were, the earliest fossils that resembled members of our species came from southern and eastern Africa. The resultant notion of a “single African origin” for modern humans stood in contrast to “multiregional” interpretations, in which the major modern geographical groups of H. sapiens were seen as having extremely deep roots in time (27). The single African origin notion received an enormous boost from molecular systematics when DNA comparisons (28, 29) began strongly supporting earlier conclusions based on proteins (30) that Africa had been the ultimate source of modern human populations worldwide.  Over the last quarter-century, evidence on both the molecular and the fossil fronts has accumulated to the point where there can be little doubt that humankind ultimately originated in Africa. 

ii.) "Beyond this, however," Tattersall continues, "the picture is a little hazy":

To some extent, the perspective is complicated by context, for although Homo neanderthalensis, for example, was clearly a member of a larger clade of species united by a suite of readily identifiable characters, the distinctive H. sapiens is largely isolated, bereft of evident close fossil relatives. This isolation was obscured for many years by paleoanthropologists' habit of designating as “archaic H. sapiens” a very motley assortment of relatively large-brained hominids that are geologically fairly recent, but that are clearly not of modern H. sapiens morphology. In Africa, fossils assigned at one time or another to this meaningless wastebasket taxon include the very distinctive crania from Ngaloba, Ndutu, Kabwe, Florisbad and Jebel Irhoud. In blurring the very clear morphological boundaries of our living species the artificial construct of “archaic H. sapiens” served to obscure a picture of considerable morphological and presumably also taxonomic hominid complexity in the later Pleistocene, and it is fortunate that most paleoanthropologists seem now to have recognized its counterproductive nature.

Fair enough, so far as it goes.  However, the evolutionary picture is further complicated by more recent discoveries in regions such as Denisova Cave in Siberia.  (See subheading 2, below.)

Relevant references cited in the previous paragraph are:

(23) McDougall I, Brown FH, Fleagle JG (2005) Stratigraphic placement and age of modern humans from Kibish, Ethiopia. Nature 433:733–736.

(24) White TD, et al. (2003) Pleistocene Homo sapiens from Middle Awash, Ethiopia. Nature 423:742–747.

(25) Bräuer G (1984) in The Origins of Modern Humans: A World Survey of the Fossil Evidence, eds Smith F, Spencer F (Alan R. Liss, New York), pp 327–410.

(26) Stringer CB, Andrews P (1988) Genetic and fossil evidence for the origin of modern humans. Science 239:1263–1268.

(27) Wolpoff MH, Wu X, Thorne AG (1984) in The Origins of Modern Humans: A World Survey of the Fossil Evidence, eds Smith F, Spencer F (Alan R. Liss, New York), pp 411–483.

(28) Cann RL, Stoneking M, Wilson AC (1987) Mitochondrial DNA and human evolution. Nature 325:31–36.

(29) Stoneking M, Sherry ST, Redd AJ, Vigilant L (1993) New approaches to dating suggest a recent age for the human mtDNA ancestor. Phil Trans R Soc Lond Ser B 337:167–175.

(30) Nei M, Roychoudhury AK (1974) Genic variation within and between the three major races of man, Caucasoids, Negroids and Mongoloids. Am J Hum Genet 26:421–443.

There is much data to be examined in these and other studies.  I do find the data compelling for an Africa origination of the genus homo.  I am not entirely convinced that all existing modern human variations can be traced precisely and linearily straight back to Africa.  Data from such regions as Denisova Cave in Siberia (see below) suggest at the least a more complicated map in the human evolutionary landscape.

Main Source:

Tattersal, Ian. 2009. Human origins: Out of Africa. PNAS 106.38. Web. http://www.pnas.org/content/106/38/16018.full

II.) DNA and Denisova Cave

Paleontological evidence from bone and teeth fragments at Denisova Cave in Siberia point to clear human habitation in the region more than 110,000 years ago.  These humans may have
originated from a line millions of years back in the African continent (they also may have evolved independently), but it is clear that along with Neanderthals in Europe and Western Asia, humans in Denisova Cave and surrounding regions had little directly to do with Africa and evidently had sufficient time to adapt to the harsh climate of Siberia.  In any case, paleontologists, anthropologists, archaeologists, molecular biologists, and geneticists will have their hands full, certainly, with clarifying these connections with the African evolution and migration hypotheses.

Note: This does not in any way deny African origins.  This is clear.  What is not entirely clear, and not entirely supported in a uniform and uncomplicated fashion, is whether or not there may have been additional lines of the human lineage that evolved concurrently if not independently from those migrating directly out of Africa.  I do not think that the evidence supports an Asian origin for the homo genus. (Tattersal's review is fairly compelling and reasonably presented.)  What may be true, however, is that human genetic diversity is greater than previously supposed and that at least some of this diversity can be attributed to independently evolved or at least divergent evolutionary lines.

Sources:

Gruber, Karl. 2013. 'Discovery of Oldest DNA Scrambles Human Origins Picture.' National Geographic. Dec. 5. Web. http://news.nationalgeographic.com/news/2013/12/131204-human-fossil-dna-spain-denisovan-cave/

(2) Reich David, Richard E. Green, et al. 2010. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature 468: 1053-1060. Web. http://www.nature.com/nature/journal/v468/n7327/full/nature09710.html?pagewanted=all

See especially remarks in Reich et al.'s (2010) conclusion:

The Denisova individual belongs to a hominin group that shares a common ancestor with Neanderthals but has a distinct population history. We define this group based on genomic evidence and call it Denisovans, but refrain from any formal Linnaean taxonomic designations that would indicate species or subspecies status for either Neanderthals or Denisovans. In our view, these results show that on the Eurasian mainland there existed at least two forms of archaic hominins in the Upper Pleistocene: a western Eurasian form with morphological features that are commonly used to define them as Neanderthals, and an eastern form to which the Denisova individuals belong. In the future, when more complete genomes from these and other archaic hominins will be sequenced from remains that allow more morphological features to assessed, their relationships will become even better understood. This will be an important endeavour as the emerging picture of Upper Pleistocene hominin evolution is one in which gene flow among different hominin groups was common.

(3) Sawyer, Susanna, Gabriel Renaud, et al. 2015. Nuclear and mitochondrial DNA sequences from two Denisovan individuals. PNAS 112.51. Web. http://www.pnas.org/content/112/51/15696

(4) Wong, Kate. 2010. 'No Bones about It: Ancient DNA from Siberia Hints at Previously Unknown Human Relative'. Scientific American. March 24. Web. http://www.scientificamerican.com/article/new-hominin-species/

(5) Zimmer, Carl. 2015. 'In a Tooth, DNA from Some Very Old Cousins, the Denisovans'. The New York Times. Nov. 16. Web. http://www.nytimes.com/2015/11/17/science/in-a-tooth-dna-from-some-very-old-cousins-the-denisovans.html

III.) All of this potentially lends additional support to the multiregional hypothesis advanced by Wolpoff et al.

See e.g.:

Wolpoff, MH, J Hawks, R Caspari. 2000. Multiregional, not multiple origins. American Journal of Physical Anthropology 112.1. Web. http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1096-8644(200005)112:1%3C129::AID-AJPA11%3E3.0.CO;2-K/abstract;jsessionid=AEDE8E9DCCF524C833B1FAFABB821EB7.f01t03

Wolpoff, MH, SH Lee. 2003. The pattern of evolution in Pleistocene human brain size. Paleobiology 29.2: 186-96. Web. http://paleobiol.geoscienceworld.org/content/29/2/186.abstract

From all of this data, I conclude the following:

A.) Humans originated in Africa, barring any discoveries of fossils in Asia or other regions that can be dated back 2 million years or more. (Note: This eventuality cannot be entirely ruled out at this point.  There is much potential fossil evidence that might be recovered in Siberia and other regional topographies.)

B.) However, it may be reasonable to speak in terms of diverging evolutionary lines in the homo genus, not necessarily traced in a direct lineation to Africa (except in the sense of ultimate origin for Hominidae, >6 million years ago). This is suggested by Neanderthal lines as well as by more recent discoveries at Denisova Cave (clear, established human habitation in Siberia ca. 110,000 years ago).

C.) Therefore, to suggest that all humans are "100% African" in their origins is simply not accurate and there is much indeed that we do not yet know or understand about the complexity and diversity of the human condition, biologically, genetically, anatomically, or otherwise.  Additional fossils will likely be unearthed, data examined, and theories will, or at least should, evolve.