Central to palaeoanthropology is the hominid fossil record. Fossils may actually be any kind of evidence of early life - ancient footprints are technically fossils, for instance - but in practice almost all hominid fossils consist of bones and teeth. This is because these are the hardest tissues of the body, and are hence most likely to escape the destruction by scavengers and weathering that is the normal fate of a dead mammal’s remains. The same destructive mechanisms also ensure that rarely is a complete hominid skeleton, or even a complete dentition, preserved. Sometimes, however, the remains of hominids and other mammals are covered by protective river muds or other such sediments before they are destroyed; and as those sediments build up into thick layers of rock the original components of the bones and teeth are replaced by minerals - which may on occasion replicate the physical features of the originals in amazingly fine, even microscopic, detail. Sedimentary rocks may accumulate in this way to great thicknesses - hundreds or even thousands of feet - but should they eventually be eroded away by water or wind there is a brief window of time in which any fossils they contain will be exposed at the surface and thereby become available for collection by palaeontologists. It is these that are the database of palaeoanthropol-ogy, and it should be evident that, for palaeo-anthropologists, being at the right place at the right time is critical. Vastly more fossils are lost to erosion than ever make it to the palaeontologist’s workbench, which is one of the many reasons why the fossil record of any kind of organism will always be highly incomplete. As a result, the practice of homi-nid palaeontology has been likened to doing a huge jigsaw puzzle with only a few of the pieces - and no picture on the box.
Fossils have three essential qualities: what they look like (broadened these days to include their microscopic and chemical qualities and in some cases their DNA content); where they come from; and how old they are. Generally, fossils are sorted into species on the basis of their external appearance (not an easy task, since there is no quantifiable degree of morphological distinction that corresponds to species difference), in a process known as ‘alpha taxonomy’. Once species identities have been established, the relationships among species are determined on the basis of their possession of ‘derived’ characteristics - those inherited from a recent common ancestor. This procedure yields a branching diagram known as a ‘clado-gram’ that specifies which species and groups of species are most closely related by recency of common ancestry. Cladograms represent testable hypotheses since the distributions of one set of characteristics can be compared to those of other characters or sets. But although cladograms establish closeness of relationship among species in terms of recency of common ancestry, they do not specify the ‘kind’ of relationship involved. There is (and in principle can be) no distinction made between the relationship between an ancestor and its descendant, and between two descendants of the same ancestor. When you start to identify ancestors and descendants, you are making an ‘evolutionary tree’, which is inherently a less testable kind of proposition. This is entirely appropriate: even if any fossil species did not necessarily have any descendants, it must have had ancestors. But in interpreting such formulations you have to bear in mind the clado-gram on which the tree was based. Finally, when you add to the tree everything you know about the functional anatomy, the ecology, the age, the inferred adaptations of a species and so forth, you have a ‘scenario’. This is, of course, inherently the most interesting kind of evolutionary proposition, but it is also the farthest from the testable base.
Palaeoanthropology is renowned as one of the most controversy-ridden of the sciences, and a major reason for this is that palaeoanthropologists have often dived in at the deep end, as it were, and started out with the scenario. This has given rise to a discipline that has too often been one of pure storytelling, your choice among scenarios depending on who is the best storyteller, rather than on the rigor with which the basic evidence is analyzed. Of course, given the scale of the unknowns (and unknowables) in a historical science such as palaeoanthropology, a certain degree of uncertainty is inevitable. Still, it is wise to remember when evaluating any palaeoanthropological scenario that palaeoanthropologists continue to disagree on how many species they are dealing with in the hominid fossil record, as well as on how the species they recognize are related to one another.
A major ingredient in creating scenarios has often been the age of the fossils concerned. Palaeoanthro-pology was born in a time when evolutionary theory was dominated by a body of thought known as the ‘evolutionary synthesis’. Under this construct, evolution was viewed as an essentially linear process, with each species merely representing an ephemeral segment of a steadily changing lineage. As long as species were thus seen more or less as links in a continuous chain running through time, the age of a fossil species could be seen as the key to its evolutionary position. Now, however, it is generally acknowledged that the evolutionary process is more typically one of natural experimentation than one of gradual refinement. Spe-ciation - the origination of new discrete species - is seen as a relatively common phenomenon, with many new species emerging, competing on the ecological stage, and as likely as not eventually going extinct. As a result, understanding evolutionary histories is now seen as a matter of analysis rather than simply of discovery. Knowing the age of a fossil hominid species is not by itself enough to tell you its evolutionary significance. Nonetheless, along with its geographical provenance its age is one of the most important attributes of any fossil, and palaeoanthropology has been revolutionized since its birth in the mid-twentieth century by the introduction of ‘chronometric’ methods, which can assign to fossils an age in years (before this, only ‘relative’ dating was available: this fossil species is older or younger than these others). Most chronometric methods, many of them described elsewhere in this encyclopedia (see Dating Methods, Overview) date the rocks in which fossils are found, rather than the fossils themselves.
World History
