Published online by Cambridge University Press: 05 August 2016
It is, however, very difficult to establish the precise lines of descent, termed phylogenies, for most organisms.– Ayala FJ and Valentine JW Evolving: The Theory and Process of Organic Evolution (7)
As understood today, human evolutionary history involves some twenty species of fossil hominins, give or take a few depending on the taxonomic scheme to which one subscribes. In the field of paleoanthropology, taxonomic questions – how to classify fossil species – are famously contentious. As Richard Dawkins comments in the Ancestor's Tale (8), there is more than one paleontology book in print entitled “Bones of Contention.” Names given to species carry implications for how species are related to one another. A species placed in the genus Homo for example, is recognized as sharing a more recent common ancestor with modern humans than a species placed in the genus Australopithecus.
Resolving species relationships is no straightforward task. That is one reason (among others) why there are so many different points of view about evolutionary relationships among hominin species. Such relationships are summarized in diagrams called phylogenies or phylogenetic trees. All methods of constructing phylogenies rely on comparing the characteristics of species, from their DNA sequences to their dental features to any aspect of their anatomy and even behavior. The most direct means of genetic comparison is through DNA, but, only in more recent hominins has it thus far been possible to extract adequately preserved DNA (see Chapter 5). For earlier species, the characteristics of teeth and bones must suffice for assessing evolutionary relationships. As noted, teeth figure prominently in such assessments: they are extremely well-preserved in the fossil record and much of the variation in their morphology is due to genes (rather than environment).
There are different ways to assess evolutionary relationships among species. According to the method of phenetics, all available traits should be used to construct phylogenies. Species with greater similarity are considered more closely related. This method does not take into account two big problems. The first is the problem of homoplasy, when two species have similar traits but not because they inherited them from a common ancestor. One way that homoplasy can occur is when species independently evolve similar solutions to similar problems. The wings of dragonflies and seagulls were not inherited from their common ancestor but evolved independently as adaptations for flight in their separately evolving lineages.