The Elements of Evolution (61-13) Lumping, Take 2

 Lumping, Take 2

We can tell species apart based on differences in the shape of their skulls, especially their face and jaws, but not on the basis of size. The differences in their skulls suggest early Homo divvied up the environment, each utilizing a slightly different strategy to survive. ~ Susan Antón

From the look of it, H. georgicus appears an intermediate between H. habilis and H. erectus. That is only the beginning of the story.

Like the H. erectus fossils that have been found, the Dmanisi specimens exhibited considerable discrepancies. One adult male skull had a large jaw and small brain case: just 546 cm – the size of Au. sediba. Others had brains up to 25% larger, but smaller jaws.

Despite that, there were underlying similarities in skull structures. All told, the Dmanisi variations appear no greater than that found in modern humans or chimpanzees.

H. erectus has been found in disparate places and dat-ed to distinct times. In contrast, the Dmanisi specimens seem anthropologically contemporaneous. Hence, despite the wide grade, that the Dmanisi hominins were a single species seems reasonable.

The 5 Dmanisi crania exhibit derived characters of H. erectus, but also retain some primitive characteristics, including small brain sizes (538 to 750 cm3), suggesting they are part of an early dispersal of that species. ~Susan Antón, Richard Potts,  & Leslie Aiello

The Dmanisi find provoked some researchers to declare that H. georgicus and H. ergaster should all be lumped in with H. erectus, as a single species. Such a sweeping conclusion alters the overarching view of human descent.

If this Homo erectus was a single species, it had a tremendous intercontinental range, especially compared with hominin primate ancestors: apes. Chimpanzees reside only across a narrow band of sub-Saharan Africa. Even in such a small range, ape species have tended to split apart.

2 MYA, the Congo River formed a bend that cut off chimps in the southern part of the range from easy access to their relations in the north. These southerners evolved into the smaller, peaceable bonobo, proving a considerable contrast to their more aggressive erstwhile kin.

Baboons tell a much different speciation story. The largest Old World monkey genus lives across much of Africa and in the Middle East. From place to place, baboon groups look so different as to easily deserve the nod to speciation. A half-dozen baboon species have remained distinct for 4 million years. Some of these different baboons have overlapping ranges. When they come into contact, they regularly interbreed, birthing healthy hybrids. Yet the troops, and species, remain separate, never blurring together. Much of this owes to different lifestyles. Some survive in the desert, while others are woodland dwellers.

The radical proposal to subsume the well-established taxa H. habilis and H. rudolfensis into H. erectus warrants careful scrutiny, and in my view the presented evidence is weak. It is doubtful whether analyses of overall cranial shape have the diagnostic power to distinguish between closely related taxa, as is indeed demonstrated by some of the analyses presented. Species are defined by specific morphological features, not by overall cranial shape. ~ English evolutionary anthropologist Fred Spoor

Taxonomic diversity sustained by ecological differentiation characterized Homo between ~2.0 and 1.4 MYA. 3 distinct lineages of early Homo in Africa remains the most compelling hypothesis. ~ Susan Antón, Richard Potts, Leslie Aiello

Leaning toward lumping may be a simplification of the dynamic of diversification and occasional melding that appears to have been typical among hominids. Fossil specimens repeatedly show a mixture of features which indicate interbreeding between distinct populations (nominally species). This practice appears to be consistent throughout hominid history.

Modern humans migrating into new regions must have interbred with the archaic populations already present in those regions. ~ American anthropologist Laura Shackelford

Outliers keep being found, leaving lumping as nondescriptive, if not problematic. Finds in Spain of 400,000-year-old hominins present a mixture of Homo heidelbergensis, Denisovan, and Neanderthal traits. It suggests that interbreeding among Homo species (or subspecies) was common.

DNA examination of Melanesians indicates an admixture which includes a hominid group for which fossils have yet to be found. Thanks to rampant interbreeding, the descent of humankind is exceedingly complex.

No one knows what to make of this. ~ Ian Tattersall

If anything, splitting would be more informatively appropriate.

Classification is complicated by the apparent fact that hominids were often extensive travelers who had sex with most everything they could. The randiness of modern males has deep evolutionary roots (not to slight lusty women).

Dupes for confusion, there are no agreed-upon criteria by anthropologists to determine speciation. Centuries of classification have yielded no sensible consensus. This is likely to continue.