Leonardo’s Mountain of Clams and the Diet of Worms (26 page)

Let me now crank up the scale for this cardinal error of linearization
one notch further—from my personal mistake about a modern nation, to a serious blunder that long delayed the explanation of an entire culture with an extended history, to a major misconception that often stymies our understanding of human evolution as a totality.

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’s extraordinary history of spread and domination. Such a statement does not, however, necessarily imply that human history—from the split, 6 to 8 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 in 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 cubic centimeters, generated five new species, all during a short and crucial period, say between 2.2 and 2.0 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 2 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 yours truly to satisfy some perverse and personal whim.

Yet 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 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.)

My friend and closest colleague, Niles Eldredge, 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 advantages of certain 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 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 does not contribute to the major progressive trends of life’s history.) Ernst Mayr, the dean of American evolutionists, and a strong supporter of copious speciation as a central ingredient in evolutionary
trends, expressed the contrast well by writing:

Nonetheless, the linear view has, until recently, strongly dominated traditional thinking about human evolution.
For example, the hoary and clichéd concept of a “missing link” presupposes linearity—for links are joining points in a sequence. Evolutionary bushes may be riddled with all the absences and uncertainties imposed by our poor fossil record, but a bush cannot feature a single and crucial “missing link.”

Moreover, the linear view has not just been accepted passively or unthinkingly—as a simple expression
of an unquestioned bias. The idea of coexistence among several hominid species has been actively denied, attacked, and even stigmatized as bad biological reasoning. For example, when I was a graduate student in the 1960s, an idea called “the single species hypothesis” still enjoyed strong, probably majority, support among students of human evolution. According to this theory, only one hominid
species could, in principle, occupy a single region at any one time. Thus, since most of our evolutionary history had unfolded on the single continent of Africa, our trends must arise by linear transformation, with only one species living at any moment, slowly perfecting itself toward the next stage. Advocates cited (I would say mis-cited) the ecological principle that only one species can occupy
any “niche”—or suitable environment for “making a living.” Beetles have “narrow” niches, so several species can live in one area—some on bark, some on the ground, some high in trees. But hominids, with our unique invention of “culture” (however primitive at first), occupy such a “broad” niche that no single place can house more than one species.

The leading basic textbook in physical anthropology
at the time (
Human Evolution
, by C. L. Brace and M. F. Ashley Montagu, 1977, first edition 1965), held that “the known fossils are most realistically placed in a linear evolutionary relationship.” The authors specified four sequential stages—australopithecine, pithecanthropine, Neanderthal, and modern—and justified their sequence by the “single species hypothesis.” They wrote:

C. Loring Brace, one of the authors of this text, has continued to resist the notion of bushiness in hominid evolution. In the 1991 edition of his popular text
The Stages of Human Evolution
, Brace acknowledges only one side branch in the entire
history of human evolution—and he calls this substantial lineage of robust australopithecines a “twig”!

Brace dismisses the idea that two (or more) human species might have interacted in one place. He even invents the label of “hominid catastrophism” to stigmatize the view (now favored by most paleontologists, particularly for the
replacement of Neanderthals by moderns in Europe) that a temporal transition from one species to another might arise by immigration of the later species from another region (followed by local extinction of the original inhabitants), rather than by linear evolutionary transformation. Brace writes:

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 represents an 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 chronological discoveries
and arguments, with the latest news as the fifth finding.

1.
T
WO 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. 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 lineage 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.
C
OEXISTENCE OF
A
USTRALOPlTHFCUS AND
H
OMO.
Linearists could still adopt a fallback 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” 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 yielded bones of African
Homo erectus
(sometimes called
Homo ergaster
, but little different from the Asian
Homo 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.
T
HE PLETHORA OF
A
FRICAN SPECIES BETWEEN
3
AND
2
MILLION YEARS AGO.
Two branches destroy the linear theory, but 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 hominid history has 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 3.0 to
more than 4 million years ago. (For most of this interval, we know only
Australopithecus afarensis
, the famous “Lucy” of our popular literature.) But between 3.0 and 2.0 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
, shows as many as six coexisting hominid species during this period, three within our own genus
Homo.

4.
B
USHINESS IN LATER HUMAN HISTORY: THE
N
EANDERTHAL 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 (though, I judge, dwindling) theory for later human history during the past million years or so, and especially for the origin of
Homo 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 will probably be remembered as 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
Homo 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
Homo 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
Homo sapiens.
Such an idea represents linearity with a vengeance—as all subgroups within a single species move onward (and brainward) in the same optimal direction. In Europe, for example,
Homo erectus
evolves
to Neanderthal, and Neanderthal transforms to
Homo sapiens
—one species at any time, but constantly on the upward move.