Bernard Wood, Dandy Doherty, and Eve Boyle
The clade (a.k.a. twig of the Tree of Life) that includes modern humans includes all of the extinct species that are judged, on the basis of their morphology or their genotype, to be more closely related to modern humans than to chimpanzees and bonobos. Taxic diversity with respect to the hominin clade refers to evidence that it included more than one species at any one time period in its evolutionary history. The minimum requirement is that a single ancestor-descendant sequence connects modern humans with the hypothetical common ancestor they share with chimpanzees and bonobos. Does the hominin clade include just modern human ancestors or does it also include non-ancestral species that are closely related to modern humans? It has been suggested there is evidence of taxic diversity within the hominin clade back to 4.5 million years ago, but how sound is that evidence?
The main factor that would work to overestimate taxic diversity is the tendency for paleoanthropologists to recognize too many taxa among the site collections of hominin fossils. Factors that would work to systematically underestimate taxic diversity include the relative rarity of hominins within fossil faunas, the realities that many parts of the world where hominins could have been living are un- or under-sampled, and that during many periods of human evolutionary history, erosion rather than deposition predominated, thus reducing or eliminating the chance that animals alive during those times would be recorded in the fossil record. Finally, some of the most distinctive parts of an animal (e.g., pelage, vocal tract, scent glands) are not likely to be preserved in the hominin fossil record, which is dominated by fragments of teeth and jaws.
Pamela R. Willoughby
In evolutionary terms, a modern human is a member of our own species, Homo sapiens. Fossil skeletal remains assigned to Homo sapiens appear possibly as far back as 300,000 or 200,000 years ago in Africa. The first modern human skeletal remains outside of that continent are found at two sites in modern Israel, the Mugharet es Skhūl and Jebel Qafzeh; these date between 90,000 and 120,000 years ago. But this just represents a short, precocious excursion out of Africa in an unusually pleasant environmental phase. All humans who are not of direct sub-Saharan African ancestry are descended from one or more populations who left Africa around 50,000 years ago and went on to colonize the globe. Surprisingly, they successfully interbred with other kinds of humans outside of Africa, leaving traces of their archaic genomes still present in living people. Modern human behavior, however, implies people with innovative technologies, usually defined by those seen with the earliest Upper Paleolithic people in Eurasia. Some of these innovations also appear at various times in earlier African sites, but the entire Upper Paleolithic package, once known as the Human Revolution, does not. Researchers have had to split the origin of modern biology and anatomy from the beginnings of modern cultural behavior. The first clearly evolves much earlier than the latter. Or does it?