Homo erectus unification?

It is difficult to resolve the “multiregional” versus “out-of-Africa” debate about the origin of modern humans on the basis of fossil evidence.  For some time, it has seemed that there were fundamental anatomical differences between earliest members of the genus Homo in Africa and those found in Asia.  The 19th century discovery by Dubois of what he called Pithecanthropus erectus ( now H. erectus) in Indonesia, set the taxonomic framework for recognising that species before early-human remains of similar antiquity (dating from about 1.8 Ma) were found in Africa.  At first regarded as H. erectus, the anatomical peculiarities of the early African remains eventually forced their reclassification as a different, perhaps ancestral species to “true erects” – H. ergaster (“Action Man”).  The fragmentary remains of the earliest Asian hominids do seem to be of this species, as do those dating to 1.6 Ma from Dmanisi in Georgia.  The lack of African fossils from the period up to about 600 ka permitted the view that H. erectus was an exclusively Asian descendant from early migrants; i.e. that there was a species divergence between Africa and Asia.  Two recent finds have cast doubt on that.

The first was of a well-preserved cranium, with associated tools and abundant mammalian remains, from the Danakil area of Eritrea (Abbate et al. 1998.  A one-million-year-old Homo cranium from the Danakil (Afar) Depression of Eritrea.  Nature, v.  393, p. 458-460), which seems to blend features of both H. erectus and H. sapiens.  The latest is claimed to be indisputably an H. erectus, and comes from the highly productive Middle Awash sediments of southern Afar in Ethiopia (Asfaw, B. et al. 2002.  Remains of Homo erectus from Bouri, Middle Awash, Ethiopia.  Nature, v. 416, p. 317-320).  The last also comes from the period around 1 Ma ago.  Such is its resemblance to Asian fossils, that there seems little point in considering any minor differences as being other than the results of the polymorphism which is so characteristic of modern humans (a view long held by the palaeoecologist, Jonathan Kingdon).  The authors also suggest that assigning earlier fossils to H. ergaster is neither necessary nor useful, for the African record now suggests that they are the early members of a lineage towards later “erects”.  The close resemblance between African and Asian “erects” does appear to indicate either repeated migration to Asia or continuous genetic contact between the two populations.

(Note  Acrimony that has no bearing on scientific debate flared up around the potentially revealing Eritrean, middle-Pleistocene sites at the annual meeting of the Palaeoanthropological Society in Denver (March 2002).  One of the members of the  University of Florence team, who discovered the site at Buia in Danakil, reported that on a recent visit local people had begun offering tools and fossils for sale.  Allegedly, the locals said they had been offered money by another team, possibly led by Randall Susman of the State University of New York.  Susman and co-workers strenuously deny offering bounties, yet have had their permit for future work withdrawn by Eritrean authorities (Dalton, R.  2002.  Hints of bone bounties rile fossil hunters.  Nature, v.  416, p. 356).  It seems hardly surprising that perceptive locals, who wrest a meagre living in one of the world’s most inhospitable places, seek to make their lives a little easier by selling what is clearly valuable enough to attract well-heeled scientists to their homeland.  Rather than allow innuendo to fog the scientific issues, it would seem wise to train people who know the area intimately to become skilled fossil hunters, and to pay them a decent wage, much as has happened in Kenya and Tanzania.)

Phyllogeography and “Out of Africa”

While 2001 was becoming the “Year of the Genome”, work continued unnoticed by the press on the growing amount of information about genetic differences between modern people in widely separated parts of the world.  Moreover, computer software developed to give more meaning to that geographic variation; the science of phyllogeography  emerged.  Analysis of genetic data, using sophisticated statistics, potentially reveals the different mutations that have appeared in widely separated populations over time, and also the degree to which genetic information entered such populations as a result of movement into them by people from far-off places.  It is a complex business, but may help resolve or reconcile the two main hypotheses about the origins of modern humans. 

The “out-of-Africa” hypothesis – launched by early work on modern humans’ genetic patterns – starts with the migration of Homo erectus from Africa to colonise Eurasia, perhaps as early as 1.8 Ma ago, thereafter to evolve separately in isolation from early Africans and perhaps one another.  Fully modern humans evolved in Africa and expanded again since about 100 ka to replace and genetically extinguish those older, non-modern populations.  The alternative view of multiregional evolution also accepts an African origin for H. erectus and its early migration outwards, but that it was followed by many genetic contacts of regional populations with Africa through continued migrations over the last 1.8 Ma.  That would allow local populations to differentiate because of the distances between them, yet gene flow between them and Africa would have maintained a single evolutionary lineage.  The many shifts in climate and sea-levels through the Pleistocene would have posed repeated stresses and opportunities for the regular migrations that this multiregional trellis model demands, hence the tenacity with which its supporters hold that view.  However, a notion of modern human populations having evolved in semi-isolation over such a long time carries inevitable connotations that many people find disagreeable.  There are political undertones in the debate that do cloud the scientific issues.

One of the supporters of the multi-regional model, Alan Templeton of Washington University, Missouri USA, has applied new statistical analyses to genetic data from mitochondrial DNA – first claimed as support for the “out-of-Africa” hypothesis – Y-chromosomes and 8 other sources of genetic information (Templeton, A.R. 2002.  Out of Africa again and again.  Nature, v. 416, p. 45-51).  His work confirms the ultimate African origin of all of us, but raises the possibility of at least two expansions out of Africa, at 600 ka and 95 ka.  Now that may seem to bring much needed support to multi-regionalism, but “again and again” is not the same as the many connections required by the hypothesis.  It is a powerful demonstration of how much remains to be done, put in context by one reviewer’s comment that genetic information from 35 individuals on a Pacific island, colonised in only the last 1000 years, is inadequate to say where all the genes came from (Cann, R.L. 2002.  Tangled genetic routes.  Nature, v. 416, p. 32-33).  In the global data used by Templeton to examine more than a million years of evolution, the groupings rely on samples from as few as 35 living individuals.

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