English
中文Introduction:
The great triumph of Charles Darwin’s theory of evolution by natural selection is its complete applicability to all life that has ever lived, and ever will live. This used to be a highly disputed point, and in fact lead to a profound professional separation between Darwin and the co-discoverer of natural selection, Alfred Russell Wallace (Darwin, 1958). Wallace agreed with Darwin on the power and far-reaching consequences of their theory, except for humans. Wallace believed in the immutability of humans, because as far as he, and much of the society at the time believed, humans were special and had been made in God’s image in their current form (Jones, 2000). Therefore, Wallace would have concluded that one could not apply the principles of human evolution to all other life, because humans did not evolve, and hominids are therefore also not related. Darwin was never comfortable with this interpretation since he insisted that the true power of their theory was in it’s all encompassing nature and applicability to all life (Darwin, 1958, Darwin, 1901).
Linear Progression:
The family Hominidae are made up of the species within Homo and the Australopithecines, and are collectively called hominids (Jones et al., 2004). There are a surprisingly small number of fossils that populate the Hominidae evolutionary tree due to a number of factors; from the sheer difficulty of finding fossils, the unsuitable environment of a forest transition into savannah for preserving fossils, and the relatively low population densities that hominids lived in, to name but a few (Lewin and Foley, 2004). Therefore it is not wholly surprising that in the original evolutionary tree for hominids there was an apparent linear progression of species from Australopithecus through to Homo sapiens (Lewin and Foley, 2004). Not wholly surprising because of a lack of contrary evidence, but much more surprising because all other species, without exception, that have had their evolutionary origins studied in detail, show adaptive radiation and a branching phylogeny, and not this linear progression. For that reason this apparent linear progression from the small bipedal and occasionally arboreal Australopithecus to the tall, ranging figure of Anatomically Modern Humans (AMH) did not ‘sit’ well with Mary Leakey, member of the most famous family in paleoanthropology, and so she and her husband Louis went in search of the fossil that would put an end to this apparent paradox of evolution (Leakey, 1984). Had they not found any other hominid remains, then the answer to the above question would be that the evolution of hominids has not got any generally applicable principles to the evolution of life in general, since it is the only example of linear progression in science. However, the Leakey’s were successful and found a hominid that could not have been directly related to Homo sapiens; Australopithecus boisei (which they named Zinjanthropus boisei) (Leakey, 1984). Australopithecus boisei had such a unique array of anatomical features that it could not have been the ancestor of any of the Homo line that leads to H. sapiens. A. boisei’s cranial features alone, including premolars and molars typical of an exclusively plant diet, as well as a large sagittal crest for the anchoring of powerful jaw muscles used for grinding vegetation, ruled it out of being directly related to AMH(Jones et al., 2004, Leakey, 1984, Lewin and Foley, 2004). This discovery, among many others, moved Hominidae away from a linear evolutionary transition, into Darwin’s correct view that evolutionary principles affect all life, and hominids are no different in this respect. This therefore allowed for the potential applicability of what can be learned from hominid evolution, to all other life.
Evolutionary Principles:
The first evolutionary principle mentioned in the above discussion is that of adaptive radiation. Adaptive radiation is where as speciation occurs, different niches are created, and species diverge or radiate from each other filling these niches to reduce competition between one another (Jones et al., 2004). Before the discovery of A. boisei there seemed to be a limited amount of adaptive radiation among hominids. However now, hominids evolutionary tree, with the addition of a few more significant discoveries, is much more consistent with a modern scientific understanding, consisting of a branching distribution over evolutionary time. One interesting discovery of hominid evolution that is applicable to all other life, is that radiation is not always successful (in fact, it is more likely to be unsuccessful, which is highlighted in the statistic that 99.8% of all species that have ever lived are now extinct (Dawkins, 2004)) and therefore often leads to evolutionary ‘dead ends’. An example of both radiation success and failure comes in the hominid family tree. On one hand, one can see sequential speciation from A. afarensis, which evolved into A. africanus, which started the Homo line for example, but on the other one also sees evolutionary dead ends like the robust australopithecines such as A. boisei (which also evolved from A. afarensis) who moved into such a narrow niche in diet, that they were unable to change and evolve when the environment changed the abundance and distribution of flora (Lewin and Foley, 2004).
Radiation is dependant on the amount of variation in phenotypic diversity (the physical and physiological traits of an organism (Campbell and Reece, 2002)). It is phenotype that natural selection acts upon, but phenotype is governed by the genotype of the individual. Therefore variation in genotype, through phenotype, determines an organism’s ability for adaptation. In the case of hominids, as already discussed, a lack of ability to adapt their phenotype away from their limited dietary specialisation was the most likely reason for the extinction of A. boisei and the other robust Australopithecines. Therefore, despite apparently fulfilling one fundamental principle of evolution; that those individuals who inherit traits which fit to their environment the best, will leave more offspring and are more likely to survive over time (Darwin, 1901), hominids teach a lesson that by only retaining a limited amount of variation, both genotypic and phenotypic, one will not last over evolutionary or geological timescales because the art of survival is a fluid and ever changing process, suffering from the so called ‘Red-Queen Effect’ whereby you must run as fast as you can to stay in the same place (Dawkins, 2004, Ridley, 1994). To contrast, the other evolutionary off-shoot from A. africanus which produced Homo habilis had a much more adaptable inheritable phenotype (specifically diet), and was therefore much better at adapting and producing offspring in that changing environment (Jones et al., 2004).
Conclusion:
The principles uncovered in the above example of the disadvantages of too much adaptive specialisation is an applicable principle to life in general, since the archaeological recorded is littered with examples of species such a A. boisei, who have become so well adapted to a particular environment and resulted in that species becoming evolutionarily stale, and therefore become extinct (Dawkins, 2004).
Adaptive radiation is a further example of how applicable the study of hominid evolution is to the evolution of life in general. The way hominids have radiated and diversified has not been different to the evolutionary radiation of other species. Some of the specifics are a little more unique, such as the effects that bipedalism has had on the whole Hominidae family. There are however, other aspects of adaptive radiation which are not so specific to hominids, such as the increased encephalisation. Work done by Robin Dunbar and colleagues for example, has shown that the increase in brain size that hominids have undergone is similar to that of all ape and monkey species, whereby brain size is a function of group size (Dunbar, 2004, Dunbar, 1996). This is one of many examples where due to the understandable interest and intensive study of our own family tree, other areas of evolutionary exploration will stand to gain from what is learned of the principles of hominid evolution.
