Figure 1.1 ► Eldridge and Gould's original diagrammatic model of punctuated equilibrium tied to rapid speciation via cladogenesis, with numerous extinctions along the way. This tempo and mode of evolution best fits the "Out of Africa" hypothesis for modern human origins. From Eldredge and Gould (1972), p. 113.

Stanley, 1978, 1979; Tattersall, 1986; Eldredge, 1989). Under this model, the many gaps in the fossil record are not merely annoying hiatuses, they are actually data: they are informing us about the tempo of evolution, that in many cases these gaps are the result of rapid speciation (rapid in geological time, that is, about 100,000 years!). Given this rapid turnover, then, the transitional forms were unlikely to be fossilized; or if they were fossilized, they are unlikely ever to be discovered, given their small population size and occupation of a restricted geographical region. While there certainly are many, many gaps in the hominid fossil record, it is perhaps the Miocene hominid record from 23 to 6 million years ago that has been most clearly shown to be characterized by a tempo and mode of evolution that best fits a punctuationist model (Cameron, in press a). This will surely prove to be the case for the hominids and hominins of the Old World, for they are marked by a sudden explosion of contemporary species, many of which appear to have left no direct descendants. This is further emphasized because the fossil record will always underestimate the number of species, and we will never have fossils representing all of the species that have ever existed.

The theory of punctuated equilibrium argues that the mode of speciation is the result of reproductive isolation at the periphery of a species' range, the emphasis being on cladogenesis as opposed to anagenesis (see Eldredge & Gould, 1972; Gould & Eldredge, 1977; Stanley, 1978, 1979, 1996; Eldredge, 1989; Gould, 2002). Cladogenesis is the splitting of a single species into two reproductively isolated or genetically distinct lineages so that species remain relatively unchanged for long periods of time, occasionally interrupted by rapid or short bursts of evolutionary change resulting in speciation. The isolation of a marginalized population results in a rapid rate of speciation, which may be accompanied by the new daughter species taking over the parent species' territory. If this does occur, it is at this stage that we find the new species within the paleontological record. The daughter species is of course much more likely to be competitively inferior to the parent species and so to become extinct; but very occasionally it may outcompete or coexist with the parent species and become successful and abundant enough to become visible to us in the fossil record, having found its own niche, distinct from that of its parent species. This tempo and mode of evolution best fits the model of evolution espoused by those who support an "Out of Africa" origin for the hominins.

Anagenesis, the alternative to cladogenesis, is slow evolutionary transformation over a long period of time within a single lineage so that an

Anatomical change

Figure 1.2 ► Evolution via anagensis, in which there is limited or no cladogenesis. One species is considered to have evolved into another through gradual evolution, resulting in "chronospecies."

Anatomical change

Figure 1.2 ► Evolution via anagensis, in which there is limited or no cladogenesis. One species is considered to have evolved into another through gradual evolution, resulting in "chronospecies."

ancestral species blends insensibly into its immediate descendants (Figure 1.2). Whether anagenesis actually exists, at least as a form of gradual change, is controversial. The existence of it depends both on whether selection pressures can remain the same over long periods of time and on there being a constant stream of mutations for selection to work on. This model of evolution and "speciation" tends to be supported by those advocating the Multiregional hypothesis for human origins. Van Valen's "Red Queen effect" assumed a pattern of evolution defined by anagenesis because it argued that a species or population has to keep changing to keep up with the changes that it wreaks in its environment (Van Valen, 1973).

It is undeniable that a species does change its environment, and keeps doing so, and there are of course other species in the same environment that are busy doing the same thing. It is arguable to what extent such changes may be progressive or may be cyclic. Perhaps the animals that cause the most havoc to their environment — after humans! — are elephants. The African savannah elephant (Loxodonta africana), the best studied of the three living species, bulldozes whole stands of trees and turns bush and forest into savannah or even desert, affecting the livelihood and abundance of the other mammals that live in the same habitat; so every year hundreds of elephants are shot in southern African game reserves and national parks, based on the premise that uncontrolled populations of elephants will destroy the whole ecosystem. Yet what sounds like a clear-cut Red Queen scenario has been challenged. On a large geographic scale, the effect may be cyclical (a stable limit cycle): The elephants eat themselves out of house and home, their populations plummet and the survivors emigrate, the vegetation recovers, the elephants increase again, the circle is closed.

If there is no sustained Red Queen effect, there is no anagenesis, at least in its traditional (gradualistic) form; or else it must depend solely on gradual, continuous nonbiological changes such as long-term, unidirectional climate or sea-level change. But these seem to have been episodic, not sustained uninterruptedly. At most there is the possibility, even likelihood, that a local environment is somewhat altered after each cycle so that the cumulative effect of a long chain of cycles is really noticeable. But this begins to stretch the concept of anagenesis as gradualism. The Red Queen, when she operates, is a downwardly directed oscillation, not an inexorable slope.

A much more likely scenario is the "Effect" hypothesis of Vrba (1980). A parent species splits into two; one of the daughters (A1) is somewhat better adapted to the changed environment than the other (51) and flourishes while B1 declines to extinction. Stasis is restored. Meanwhile the environment continues to fluctuate and undergo its stable limit cycles, but the extremes of the cycle change directionally over time — the open-country phase of the cycle gets more open over time; when the forest returns, it is less dense or less widespread. After some time, A1 itself spe-ciates. Of the two daughter species, A2 is the one better adapted to the now-changed environment, and it flourishes in its turn while B2 declines to extinction. And so it goes on. Over a long period of time, the differential survival of the daughter species that each time is better adapted to the now-changed environment is the one that survives, and the effect mimics anagenesis. In the main, the fossil record is too coarse-grained to differentiate the two processes, and prior to 1980, evolutionists would assume that it was anagenesis that was taking place. Maybe it was not.

The importance of speciation has been promoted many times in the fossil record. Groves (1989a) argued that, if it is true that evolutionary change is concentrated at the point of speciation, we can predict that, of two sister species, the one that is more changed (highly autapomorphic) from the common ancestor will have undergone more cladogenesis (its lineage has gone through more speciation events) than the one that is less changed. Unfortunately, the record of human evolution offers only a partial test of this. The human species is much more different than is the chimpanzee from our common ancestor, and the human fossil record is certainly enormously speciose, but the chimpanzee fossil record is empty. All we can do is predict that, when paleontologists start prospecting in the right place to find proto-chimpanzees, they will not be very speciose. Chimpanzee evolution will prove to be, let us say, as nearly unlinear in reality as human evolution was held to be up until the 1970s, when the single-species model finally became untenable. But, as we will see presently, the single-species hypothesis has reared its head again, though not through an analysis of fossil material but, rather, by an abstract discussion of the molecular evidence.

If any statement regarding our own origins is correct, it is that humans originally evolved in Africa. We can all trace our prehistoric roots back to the African continent around 6 million years ago. It was at this time that populations of proto-chimpanzees and proto-humans split from a common ancestor and each started its own evolutionary journey. The recently described fossils allocated to Sahelanthropus from Chad, dating to between 6-7 million years ago, and Orrorin from Kenya, dating to around 6.1-5.8 million years ago, are close to the point of separation (Brunet et al., 2002; Senut et al., 2001; Pickford et al., 2002), as is the earlier hominid discovery from Lothagam, dated to between 5.0-5.2 million years ago (see M.G. Leakey & Walker, 2003).

Following on from these late Miocene genera comes Ardipithecus, which occurs at the Miocene/Pliocene transition of Ethiopia between 5.8 and 4.4 millions of years ago (Ma) (T.D. White et al, 1995; Haile-Selassie, 2001; White, T.D. 2002). Ardipithecus displays a mixture of features, some of which are chimpanzee-like while others are human-like. What traditionally marks Ardipithecus as being on the human line is that they, unlike chimpanzees, seem to have walked upright. It is from Ardipithecus or an Ardipithecus-like hominid that the later proto-australopithecines are thought by most to have emerged (Figure 1.3).

The proto-australopithecines are represented by a number of species commonly allocated to the genus Australopithecus even though they do not form a monophyletic group, meaning that they do not share an exclusive

H. erectus

0 0

Post a comment