We may legitimately speak of "general trends" in human evolution. We can also scarcely doubt that increasing brain size represents both a major trend and the key to our species' extraordinary history of spread and domination. Such a statement does not, however, necessarily imply that human history - from the split, six to eight million years ago, of our ancestors from the common stock that also generated our closest cousins (chimps and gorillas) to our current, exalted state - should be interpreted as a linear series of advancing steps in brain power, with any stragglers or groups that failed "to go with the program" relegated to extinction as side branches on an inevitable cul-de-sac, or dead end.
Many paths and mechanisms can lead from a small-brained beginning to a top-heavy current status. To cite the most radical evolutionary alternative to the traditional linear view - a false extreme, to be sure, but providing as much partial insight as the equally erroneous linear alternative suppose that an ancestral Species A, with an average brain volume of 300 cc, generates five new species, all during a short and crucial period, say between 2.2 and 2 million years ago. These five species arise with different average brain volumes B at 500 cc, C at 700, D at 900, E at 1,100,, and F at 1,300 - and do not alter these figures during their geological lifetimes. All six species (A and the five descendants) live for two million years with no further change. (,They may never even come into direct competition, for each may inhabit a different continent - the result of A's rapid spread around the world and equally quick evolution to B, C, D, E, and F in five separate areas.) Finally, species A through E become extinct and only F survives. We call F - Homo sapiens.
In both extreme cases, human ancestors
begin at 300 cc, and peak today at 1,300 cc. Both schemes invoke a metaphor
of struggle and persistence the slow climb up a ladder in the traditional
linear view, and success in hanging on through every adversity in the "bush
pruning" alternative. Both views are also clearly wrong in their exclusive
versions. Why, then, do we tend to feel comfort and affinity for the linear
;scheme, while regarding the "bush pruning" alternative as laughable
and inexplicable nonsense, no doubt introduced by your resident essayist
to satisfy some perverse and personal whim?
I wish to argue (1) that both views express important partial truths; (2)
that we have favored the linear view primarily, in obedience to the disabling
cultural bias illustrated by the earlier examples in this essay; (3) that
the history of twentieth-century ideas about human evolution can be epitomized
by the growing
strength of the "bush making and pruning" view and the retreat
of the linear view all leading to a proper balance; and (4) that a new discovery,
announced in December 1996 (and inspiring this essay), provides strong and
unexpected support for bushiness as the usual condition of the human lineage.
(I shall, for the rest of this essay, refer to the two modes of thought
as "linear" and "bushy" accounts of evolutionary trends.)
The invertebrate paleontologist Niles Eldridge, my friend and closest colleague;
has labeled these - two approaches to trends as "taxic" and "transformational"or
"based on the production of many separate species" (formally named
groups of organisms, such as species and genera, are called "taxa")
versus "propelled by the Homo advantages of certain 4 traits"
(big brains, for example) in competition among varying individuals within
a single group. The two views differ most significantly in their primary
"motors" for generating trends. In the bushy, or taxic, theory,
trends require a substantial production of independent species, for the
net change in a lineage depends upon a differential survival and further
proliferation of some species versus the extinction of others. In the linear,
or transformational, theory, trends require no bush of species but arise
by the competitive success of favorable traits in a gradually progressing
unit. (Of course, supporters of the linear view do not deny that lineages
may also produce new species by branching, but these scientists tend to
separate the progressive carrier of the trend from doomed side branches.
In other words, for the linear transformationist, the production of numerous
species is irrelevant to the major progressive trends of life's history,
however interesting as an evolutionary phenomenon.)
Ernst Mayr, the dean of American evolutionists, and a strong supporter of
copius speciation as a central ingredient in evolutionary trends, expressed
the contrast well by writing in Animal Species and Evolution:
I feel that it is the very process of creating so many species which leads to evolutionary progress. Species, in the sense of evolution, are quite comparable to mutations. They also are a necessity for evolutionary progress, even though only one of many mutations leads to a significant improvement of the genotype.
. . . Seen in this light, it appears then that a prodigious multiplication species is a
prerequisite for evolutionary progress. . . . Without speciation, there would be no diversification of the organic world, no adaptive. radiation, and very little
evolutionary progress. The species, then, is the keystone of evolution.
Nonetheless, the line view has, until recently strongly dominated traditional
thinking about human evolution......Yet of all alterations in thinking about
human evolution that have occurred during my professional lifetime, none
has been more transforming, or further ranging in implications, than the
increasing documentation of substantial bushiness throughout most of hominid
history. Our present reality of one worldwide species is the oddity, not
the norm and we have been fooled by our bad habit of generalizing a transient
and contingent present.
I would summarize this fundamental change from the linear to the bushy view of our evolutionary history in five chronologically ordered discoveries and arguments, with the latest news as the fifth finding.
1. Two branches of australopithecines: When South African scientists described Australopithecus the genus ancestral to our own, Homo in the 1920s they designated two major branches or species, Australopithecus africanus and A. robustus (known in later literature as the gracile and robust forms). Thus, a bushy theory for our early days enjoyed-some support from the start of modern research. But proponents of the single species hypothesis either viewed the two names as improperly given to males and females of a single species, or (as in the quote from Brace previously cited) regarded the robust species as a doomed and insignificant side branch, probably driven to extinction by our superior forebears, the graciles.
However in 1959, Mary Leakey found a key specimen with robust features so exaggerated that sexual variation within a single species became implausible as an explanation for the extent of difference. The probable coexistence of two australopithecine lines could no longer be denied and the purest version of the single species hypothesis died. (Mary Leakey originally called this skull Zinjanthropus; we now generally designate this form as a separate, so-called hyper-robust species, Australopithecus boisei.)
2. Coexistence of Australopithecus and Homo. Linearists
could still adopt a fall back position. They could brand the robust (and
hyper robust) australopithecines as an insignificant blind alley,, regard
the graciles as linearly ancestral to our own genus, Homo and then
apply the single species hypothesis to Homo alone, drawing a line
from Homo erectus Java man and Peking man in the older texts) through
Neanderthal to our current exaltation. But then, in the mid-1970s, Richard
Leakey (Mary's son) found hyper-robust specimens in the same strata that
had yielded bones of African H. erectus (sometimes called H. ergaster,
but little different from the Asian H. erectus of Indonesia and
China). No one could possibly encompass this range of variation within the
boundary of a single species. If the most extreme of the robust australopithecines
coexisted
with the most advanced members of our own ancestry, then the
old line of
progress had become an undeniably diverging bush.
3. The plethora of African species between 3 and 2 million years ago. Two branches destroy the linear theory, but they don't build a very impressive bush. In the twenty years since Richard Leakey's discovery of these two irrefutably coexisting species, further research on The "brand hominid history has 1.5 million stressed one primary theme above all others: the bush gets bushier and bushier. To summarize a great deal of elegant research in too short a statement: We have no evidence for more than one species during the earliest period from 3 to more than 4 million years ago. (For most of this interval, we only know A. afarensis, the famous "Lucy" of our popular literature see my essay of September 1994.) But between 3 and 2 million years ago (and mostly during the last half million years of this interval), a virtual explosion of hominid species occurred on both major branches of the hominid bush that is, both within the ancestral genus Australopithecus and within the derived genus Homo. The accompanying chart, presented in Donald Johanson and Blake Edgar's recent book From Lucy to Language (Nevraumont/Simon and Schuster, 1996), shows as many as six coexisting hominid species during this period, three within our own genus, Homo.
4. Bushiness in later human history: The
Neanderthal issue. Linear preferences die hard. I think that all major students
of the subject now accept substantial bushiness and coexistence of several
species in Africa, during early hominid history, but a version of the old,
linear view still persists as a popular (although, I judge, dwindling) theory
for later human history during the past million years or so, and especially
for the origin of H. sapiens. This debate has been prominently featured
in the press (and treated in several of these essays) as a conflict between
the ""Multiregional" and "out-of-Africa" theories
for modern human origins. Multiregionalism is the last post of the linear
view. Under this model, all hominid evolution occurs in Africa (admittedly
in a fairly bushy manner) until the origin of H. erectus. This species
then spreads out to all the Old World continents between 1.5 and 2 million
years ago. The three major populations of H. erectus, in Africa,
Europe, and Asia, then evolve in parallel (abetted by a low level of migration
and consequent mixing among the three groups) toward H. sapiens.
This is linearity with a vengeance as all sub-groups within a single
species move onward (and brainward) in the same optimal direction. In Europe,
for example, H. erectus evolves to Neandertal, and Neanderthal transforms
to H. sapiens one species at any time, but constancy on
the upward move.
The out-of-Africa alternative may best be understood as a particular version
of the bushy perspective. H. erectus moves out to all three Old World
continents. H. sapiens arises as a branch (the bushy view)
from one of these populations, not as a terminus to a universal trend. H.
sapiens then spreads as a second diaspora from its place of origin,
presumably Africa on both genetic and paleontological grounds. But H.
erectus (or its descendants) already inhabit Europe and Asia, so African
H. sapiens arrives as a second human species (bushy coexistence again)
and eventually supplants the original form. Under this bushy view, Neanderthals
and modern H. sapiens are separate (and potentially coexisting human
species, not the before and after of a single linear transformation:for
Neanderthal branched from European H. erectus (or its descendants),
while forebears of modern Europeans arrived from Africa after a separate
origin from African H. erectus populations.
In my reading, and as summarized elsewhere (perhaps best in the recent book by C. Stringer and R. McKie, African Exodus: The Origin of Modern Humanity, Henry Holt,, July 1997), the balance of recent evidence tilts strongly (perhaps now even conclusively) to the out-of- Africa view, and therefore to the predominance of bushiness over linearity as a central theme in human evolution.....
5. If Neanderthal and H. sapiens coexisted as independent species in Europe, thus refuting the linear view, what happened in eastern Asia, where Eugene Dubois first discovered H. erectus in the 1890s, and where this ancestral species enjoyed long and widespread success? In the multiregional view, these Asian H. erectus populations evolved directly into modern Asian groups of H. sapiens. In the bushy alternative, H. sapiens arrived (ultimately from Africa) as a second wave of migration, and may have coexisted for a time with Asian H. erectus or its descendants.
The obvious test between these starkly different views requires a fossil record, either of intermediacy or of coexistence, during the crucial time of transition between the two species. But such decisive data have not been available because the youngest known Asian H. erectus (from China) range from about 290,000 to 420,000 years old, while the oldest Asian H. sapiens specimens are only about 40,000 years old. Thus, we had no evidence at all for the crucial intervening years.
Dubois first discovered H. erectus in Java during the early 1890s and these specimens, from Trinil, remain the most famous Indonesian representatives of the species. But in the early 1930s, Dutch geologists discovered a suite of twelve hominid calvariae (skull tops lacking the facial skeleton and upper jaws) from the nearby site of Ngandong on the banks of the Solo River. These specimens variously known in the older literature as Solo man, or H. soloensis have engendered a long and substantial debate about their identity, but a present consensus considers them as members of H. erectus, the species Dubois found.
Yet while anthropologists had finally reached some agreement about their identity, the age of the Solo specimens remained unknown. This crucial issue may now have been resolved in a surprising manner by an article that appeared in the December 13, 1996, issue of Science magazine: "Latest Homo erectus of Java: Potential contemporaneity with Homo sapiens in Southeast Asia," by C. C. Swisher III, W.J. Rink,