If you happen to live in the Australian Capital Territory1, you live in a relatively small enclave excised from New South Wales. It is a subcrescentic area of land with a border that is just over 300 km long. If you stand on the northern extremity of the territory and your mother is a metre away to your right, and her mother (your grandmother) a metre to her right, and her mother (your great grandmother) a metre to her right, and so on all around the border, you will go through about 300,000 generations and the being standing on your left, as the loop is completed, will be just over a metre tall, covered in thick hair and may be at least partly arboreal. Given that it is likely that age at reproduction was considerably less that many generations ago than for modern humans2, it is likely this 300,000th ancestor would have lived about 3.5-4.5 million years ago. At that time the only hominins on our line of ancestry were gracile australopithecines such as Ardipithecus ramidus and Australopithecus afarensis3.
As you can see, these hominins have different species names to ours (sapiens) and they also have different generic names to ours (Homo). However, your mother belonged to the same species as you, and your grandmother belonged to the same species as your mother, and your great grandmother belonged to the same species as your grandmother. And this goes for every ancestor-descendant couple in these 300,000 generations. So what are we to make of the assignment of such small hairy beings to different genera and species to ours? It is purely an example of two phenomena, the human propensity to categorise things, and the relative paucity of fossil hominin specimens. As we find more and more fossils the distinctions between species become finer, and genera will proliferate.
This brings me to the fossils, reputedly of Homo sapiens, which have been found at Jebel Irhoud in western Morocco. These fossils have been dated to a little over 300,000 years, which places them at about 100,000 years older than the previous oldest specimens of H. sapiens from east Africa4. Parts of five individuals were recovered, and these indicate that the faces were much like modern humans in being relatively flat without the pronounced prognathous forward projection of earlier hominins, but with a less prominent chin, and with a steeply sloping forehead. The Moroccan specimens do have prominent brow ridges, but the forehead does not slope as strongly backwards from the brow ridges as in Neanderthals or H. erectus. One thing which has puzzled the discoverers slightly is that the skull is much more elongate than the bulbous modern human skull. This is interpreted to indicate that the brain was organised differently to that of modern humans4, perhaps more like that of Neanderthals.
As an indication of the problematic assignment to species, there is some doubt as to the assignment to H. sapiens, with Jeffrey Schwartz (University of Pittsburgh) claiming that too many different-looking fossils have been lumped together in the species, thus complicating efforts to interpret new fossils. In addition, María Martínon-Torres (University College London) has suggested that the lack of a prominent chin and a flat forehead should preclude assignment to H. sapiens4.
As more fossils are discovered, the ability to assign them to a species will become more difficult, and that is because a species can only really be defined at an instant in time. Add the fourth dimension and it all becomes fairly arbitrary, as in the thought experiment above, where you would need to decide where among your maternal lineage, the species Homo sapiens, to which you belong, ends and its predecessor species begins. That would entail drawing a boundary between a mother and daughter, and assigning them to different species, a clearly ludicrous proposition. It was Linnaeus who, in the 1700s, developed the binomen of generic and specific names to distinguish one species from another. Maybe Linnaeus got it wrong.
Sources
- https://en.wikipedia.org/wiki/Australian_Capital_Territory
- http://science.sciencemag.org/content/322/5904/1040.full
- https://en.wikipedia.org/wiki/Timeline_of_human_evolution
- https://www.nature.com/news/oldest-homo-sapiens-fossil-claim-rewrites-our-species-history-1.22114
