|Find the human! Pretty easy, right? RIGHT??|
But this clarity vanishes as soon as we look at the fossil record. There's a gradient of forms between us and things that are not clearly closer to us or chimpanzees (Ardipithecus, Orrorin, Sahelanthropus). Which ones are "human" and which are not? Is Praeanthropus afarensis human? What about Homo habilis? Homo ergaster? Neandertals? Homo sapiens idaltu?
|Find the human! Or is there more than one?|
Or are they all human?
This issue crops up for all kinds of taxa. Much time has been spent arguing what is and is not e.g., avian, or mammalian. The issue is more common within vertebrates than many other taxa, since vertebrates have an especially good and well-studied fossil record. But it applies, in theory or practice, to every extant taxon.
I subscribe to the school of thought that names born from neontology (the study of extant organisms) are best restricted to the crown group (that is, to the living forms, their final common ancestor, and all descendants of that ancestor). Arguments for restricting common names to crown groups were first laid out by de Queiroz and Gauthier (1992). The primary reason for doing this is that it prevents unjustified inferences about stem groups (that is, the extinct taxa which are not part of the crown group, but are closer to it than to anything else extant). For example, we currently have no way of knowing whether the statement, "Within all mammalian species, mothers produce milk," is true if we include things like Docodon as mammals (or, as a few have done, even earlier things like Dimetrodon). However, if we restrict Mammalia to the last common ancestor of monotremes and therians (marsupials and placentals) and all descendants of that ancestor, then the statement unambiguously holds.
This system also gives us a very easy way to refer to any stem group: just add the prefix "stem-". Some examples:
- stem-avians: Pterodactylus, Iguanodon, Diplodocus, Eoraptor, Coelophysis, Tyrannosaurus, Oviraptor, Velociraptor, Archaeopteryx, Ichthyornis
- stem-mammals: Casea, Dimetrodon, Moschops, Cynognathus, Docodon
- stem-whales: Indohyus, Ambulocetus, Pakicetus, Basilosaurus, Dorudon
- stem-humans: Ardipithecus(?), Praeanthropus, Australopithecus, Homo habilis, Homo ergaster
- de Queiroz & Gauthier (1992). Phylogenetic taxonomy. Annual Review of Ecology and Systematics 23:449–480. [PDF]
- Green & al. (2010). A draft sequence of the Neandertal genome. Science 328:710–722. doi:10.1126/science.1188021
- Krause & al. (2010). The complete mitochondrial DNA genome of an unknown hominin from southern Siberia. Nature 464(7290):894–897. doi:10.1038/nature08976
- Reich & al. (2010). Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature 468:1053–1060 doi:10.1038/nature09710
- Teschler-Nicola & al. (2006). No evidence of Neandertal mtDNA contribution to early modern humans. Pages 491–503 in Early Modern Humans at the Moravian Gate. Springer Vienna. doi:10.1007/978-3-211-49294-9_17