910 THE STRUCTURE OF EVOLUTIONARY THEORY
(Needless to say, no true consensus exists in this most contentious of all scientific
professions—an almost inevitable situation, given the high stakes of scientific
importance and several well known propensities of human nature, in a field that
features more minds at work than bones to study. Nonetheless, despite endless
bickering about details, I don't think that any leading expert would now deny the
theme of extensive hominid speciation as a central phenomenon of our
phylogeny—see Johanson and Edgar, 1996.)
Second, the crucial period of 2-3 million years ago, spanning the origin of
initial diversification of the genus Homo, also represents the time of maximal
bushiness for the hominid clade, then living exclusively in Africa. The
correspondence of a time of maximal speciation with anatomical change of greatest
pith and moment in our eyes—that is, the origin of our own genus Homo, with an
extensive expansion of cranial capacity—probably records a causal process of
central importance in our evolution, not just an accident of coincidental correlation.
Vrba's turnover-pulse hypothesis, an extension of punctuated equilibrium (see pp.
918 - 922), represents just one causal proposal for this linkage. As many as six
hominid species may have coexisted in Africa during this interval, including three
members of the genus Homo.
Third, the central theme of bushiness persisted far longer than previous
conceptions of human evolution had ever allowed, right to the dawn of historical
consciousness. Under earlier anagenetic views, European neanderthals marked a
transitional stage in a global passage. But under speciational reformulations, and
acknowledging extensive anatomical distance between neanderthals and moderns,
Homo neandertalensis must be construed as a European offshoot of local Homo
erectus populations, with Homo sapiens evolving in a separate episode of
speciation, probably in Africa on strong genetic and more tenuous paleontological
grounds, and then replacing neanderthals by migration. Similarly, Asian Homo
erectus populations may not have passed anagenetically into Homo sapiens, but
may also have been replaced by migrating stocks of Homo sapiens, originally from
Africa. If the redating of the Solo Homo erectus specimens can be confirmed (as
mentioned above), then this Asian replacement occurred at about the same time as
the death of neanderthals in Europe.
This information implies the astonishing conclusion, at least with respect to
previous certainties, that three human species still inhabited the globe as recently
as 40,000 years ago—Homo neanderthalensis as the descendant of Homo erectus
in Europe, persisting Homo erectus in Asia, and modern Homo sapiens, continuing
its relentless spread across the habitable world. This contemporaneity of three
species does not match the richness of an entirely African bush with some half a
dozen species about 2 million years ago, but such recent coexistence of three
human species does require a major reassessment of conventional thinking. The
current status of our clade as a single species represents an oddity, not a generality.
Only one human species now inhabits this planet, but most of hominid history
featured a multiplicity, not a unity—and such multiplicities constitute the raw
material of macroevolution.