Punctuated Equilibrium and the Validation of Macroevolutionary Theory 909
separate taxa) inspired dubious proposals about sexual dimorphism. The same
theoretical constraint led many researchers to regard European neanderthals as
necessarily transitional between Homo erectus and modern humans, even though
the empirical record indicated a punctuational replacement in Europe about 40,000
years ago, with no evidence for any anatomical intermediacy.
I freely confess my partisan attitude, but I do think that reforms of the past
two decades have centered upon a rethinking of this phylogeny in speciational
terms, with punctuated equilibrium acting both as a spur for reformulation and a
hypothesis with growing empirical support. Claims for stasis have been advanced,
and much debated, for the only two species with appreciable longevity of a million
years or more—Australopithecus afarensis, aka "Lucy," and Homo erectus
(Rightmire, 1981, 1986, for support; Wolpoff, 1984, for denial). Lucy's earlier case
seems well founded, while claims for our immediate ancestor, Homo erectus, have
inspired more controversy, with arguments for gradual trending towards Homo
sapiens, at least for Asian populations, generating substantial support within the
profession. (But if the reported date—see Swisher et al., 1996 —of 30,000 to
50,000 years for the Solo specimens holds, then the geologically youngest Asian
H. erectus does not stand at the apex of a supposed trend.)
More importantly, hominid "bushiness" has sprouted on all major rungs of the
previous ladder implied by the single-species hypothesis. The hominid tree may
grow fewer branches than the comparable bush of horses, but multiple events of
speciation now seem to operate as the primary drivers of human phylogeny (see
Leakey et al., 2001, for a striking extension to the base of the known hominid bush
in the fossil record), while humans also share with horses the interesting feature of
present restriction to a single surviving lineage, albeit temporarily successful, of a
once more copious array. (I would also suggest, on the theme of "life's little joke,"
that the contingent happenstance of this current restriction has skewed our thinking
about human phylogeny away from more productive scenarios based on
differential speciation.)
Speciation has replaced linearity as the dominant theme for all three major
phases of hominid evolution (see Tattersall and Schwartz, 2000; and Johanson and
Edgar, 1996, for booklength retellings of hominid history centered upon this
revisionary theme of bushiness vs. linearity). First, before the origin of the genus
Homo, the australopithecine clade differentiated into several, often
contemporaneous, species, including, at a minimum, three taxa of "robust"
appearance (A. boisei, A. robustus, and A. ethiopicus), and at least two of more
"gracile" form (A. afarensis and A. africanus). Some of these species, including at
least two of "robust" form, survived as contemporaries of Homo. Historically
speaking, the death of the single species hypothesis may be traced to Richard
Leakey's discovery, in the mid 1970's, of two undeniably different species in the
same strata: the most "hyper-robust" australopithecine and the most "advanced"
Homo of the time (the African form of Homo erectus, often given separate status
on cladistic criteria as Homo ergaster).