common ancestor (discussed in detail in Chapter 5). Given their distinct evolutionary histories, they cannot be allocated to the same genus, at least not to a genus that does not include modern humans too; rather, they represent a pattern of hominin diversity, each eventually leading to extinction. Following the scheme proposed by Strait et al. (1997), Strait and Grine (2001), and Cameron (in press b), we agree that "A." anamensis, "A." afarensis, and "A." garhi either represent distinct genera (Cameron's preference) or, like all Plio-/Pleistocene hominids, should be subsumed into Homo (Groves's preference). Recently, Strait et al. (1997) and Strait and Grine (1998, 2001) have reallocated "A." afarensis (which contains the famous "Lucy" skeleton) to the genus Praeanthropus. This genus was first described in the 1950s (see also Harrison, 1993). Thus they and Cameron would argue that only one species, A. africanus (the type species), exists within the genus Australopithecus.

The evolution of the later, more derived hominins, Paranthropus, the "rudolfensis group" (represented by the famous 1470 skull), and early Homo, appear to be distinct from that of the proto-australopithecines, suggesting that these lineages have a relatively longer history than currently recognized. There are two candidates for the last common ancestor of these later hominins: Australopithecus africanus and Kenyanthropusplatyops (see Dart, 1925; M.G. Leakey et al., 2001; D.E. Lieberman, 2001; Cameron, in press a & b). Indeed, it is likely that both of these "basal hominins" branched off the line before the emergence of the proto-australopithecines. It is possible that their success occurred at the expense of the proto-australopithecines in competition for available resources. Species of Paranthropus, early Homo, and the "rudolfensis group" occupied the same habitats in time and space, so some form of competition must also have occurred between these various groups in the African forests and savannas. If the earliest representatives of Homo had succumbed to the competitive pressures of these other groups, then the world as we know it would be very different indeed!

Homo represents the first hominin to disperse out of Africa (though we will see in the next chapter that the original hominid "out of Africa" occurred during the early/middle Miocene transition). Species of Homo were in both far southeastern Europe (Georgia) and Asia (Java) by 1.6 million years ago, while Kenyanthropus, Paranthropus, and members of the "rudolfensis group" remained restricted to Africa (Strait & Wood, 1999; Gabunia et al., 2000a; Dunsworth & Walker, 2002). About 1 million years before Homo was extending its range outside of Africa, K. platyops disappeared from the fossil record. By the time early Homo were occupying a number of diverse habitats in Africa, Europe, and Asia, the last relict Paranthropus populations disappeared from the fossil record.

Later Homo were a diverse lot: Those populations from different parts of the Old World can all be distinguished easily from one another based on a number of distinct facial features. Some authorities (the "Out of Africa" school) regard them as belonging to a number of different species (Homo erectus in Java, Homo pekinensis in China, Homo heidelbergensis in Africa and Europe). It is true that a general likeness of skull shape is maintained over vast eons of time — hundreds of thousands of years — within each of these regions, though this is to be expected given the similar rate of encephalization. Only in Europe, however, was there a measurable change within one of these species: After about 400,000 years ago, Homo heidel-bergensis, which had entered Europe from Africa a few hundred thousand years before, had by 120,000 years ago become Homo neanderthalensis, the famous Neanderthal people (Stringer, 1989, 1994; Stringer & McKie, 1996), whereas the deme that remained in Africa had by 160,000 years ago emerged into near modern H. sapiens, as defined by the recent significant discoveries of the Herto specimens from Ethiopia (T.D. White et al., 2003; Clark et al., 2003; see also Stringer, 2003).

It has also been suggested by some, however, that the lineage leading to H. neanderthalensis had already been established as early as 780,000 years ago, as represented by the hominins from Atapuerca (Gran Dolina), Spain, sometimes referred to as H. antecessor (Bermudez et al., 1997). They suggest that H. heidelbergensis was already a part of the Neanderthal lineage, and as such the African hominins usually allocated to the same species must be a different species because they are not part of the Neanderthal lineage. Thus a separate and parallel line in Africa (H. rhodesiensis?) may have led to the evolution of H. sapiens via African populations, as represented by the Herto, Elandsfontein, and Kabwe specimens (see Stringer, 1998, 2003; Clark et al., 2003; T.D. White et al., 2003), so having nothing to do with the emergence of the Neanderthals.

Other authorities (multiregionalists) disagree with these interpretations. These are not different species, they say, but races of early Homo sapiens; just as modern Homo sapiens has somewhat different geographic varieties, which we sometimes refer to as "races," so did ancient Homo sapiens (Wolpoff, 1989, 1999; Wolpoff & Caspari, 1997; Wolpoff et al., 1984, 2001). This minor semantic difference makes all the difference. If they were different species, then they were genetically discontinuous, and if there was any interbreeding between them it was marginal, and their distinct genetic makeup remained unaffected. If they were demes ("races") of the same species, then they were fuzzy at the edges, and new genes from one of them would flow easily into the others.

Despite what some molecular biologists might say, fossils are still the most informative pieces of information available to us when trying to interpret evolutionary relationships among extant and fossil species. They enable us to recognize distinct and common anatomical features, which provide clues to the evolutionary relationship between the species being examined and other fossils and living organisms. Fossils also enable us to identify adaptive strategies employed by these extinct organisms. For example, the identification of large robust mandibles and molars (marked by hyperthick molar enamel) in Paranthropus species suggests that they consumed very tough food types (Tobias, 1967; Rak, 1983; Hylander, 1988; White, 2002). Using the bones and the archaeological record, we can identify, through time, how species evolved as a result of their environmental conditions and how they adapted to take advantage of new opportunities.

The study of fossils is largely an anatomical pursuit. Paleontologists spend much of their time examining fossils and comparing them to other fossils and to living organisms thought to share a close evolutionary relationship. One of the most important keys in the reconstruction of evolutionary relationships between species is the identification of polarity — those anatomical features that are primitive and those that are derived.

Primitive features are characters that are often commonly observed and widespread and are considered to have evolved at a very early stage in the group's evolution. Derived features are characters that are less widespread, often unique to a particular group, and so are likely to have evolved only recently in that group. For example, quadrupedal locomotion is a primitive character of the primates (we know this because almost all other mammals are quadrupedal), which tells us little about the evolutionary relationships within this large group. Habitual bipedal locomotion, however, is a derived feature linking humans and the proto-australopithecines and their immediate ancestors, to the exclusion of most other primates (see next chapter). In summary, fossils enable us to identify evolutionary relationships among species and likely physical adaptive trends through time and space.

Stone tools, and an interpretation of their immediate context, are an important source of information when trying to reconstruct past human behavior and cultural evolution. While early humans undoubtedly used other materials (such as wood and animal skins), these are not usually preserved in the archaeological record. The development of ever more sophisticated stone "tool kits" by early humans enabled them to adapt more readily to and extract new food resources from their ever-changing environments and habitats. It also allowed them to defend themselves from much larger and more ferocious animals, and it enabled them to hunt and thus to develop an increased sense of community. In developing this technology, early humans started their long journey on the road to reshaping their environment, rather than simply being shaped by it. Through time, a number of different tool traditions were developed. Archaeologists have been able to associate some of these tool traditions with particular human groups (Bordes, 1950, 1961, 1969; Bordes & Sonneville-Bordes, 1970; Foley & Lar, 1997), while other tool kits are clearly designed for specific functions and not related to differing "cultural" traditions (Binford & Binford, 1966; Binford, 1983). Interpreting how these tools were used has enabled archaeologists to help reconstruct aspects of past human behavior.

The recent application of molecular biology to human evolutionary studies has greatly influenced current interpretations of human origins. Our genes contain all of the relevant information pertaining to our genetic makeup; they are the core of our being (Figure 1.4). These genes are made

Every person comprises around 100 trillion cells

Each chromosome contains packed strands of DNA

Every cell has a nucleus

Every cell has a nucleus

0 0

Post a comment