The human genome consists of about 3.2 billion base pairs. By way of comparison, the lily genome comprises about 91 billion base pairs — about 30 times as much DNA as the human genome. Only about 1% (30 million) of the base pairs of the human genome are expressed as proteins, which means that humans have about 20,000 genes, with an average of 1500 base pairs per gene. Again, by way of comparison, this is about the same number as in the genomes of flatworms and diatoms (single-celled eukaryotic algæ). Further, about 8% of human DNA is retroviral material accumulated in the species through infection.
The two species of chimpanzee genomes similarly consist of about 3.2 billion base pairs. Human and chimpanzee genomes differ only in about 40 million of the base pairs — about 1 in 75 or 1.3%. That is, humans and chimpanzees share 98.7% of their DNA. Chimpanzees are more closely related to humans than they are to any other apes. The human and chimpanzee lineages split from each other only about 6 million years ago, though distinctly human traits like bigger brains and longer gestation appeared only much more recently, about 1 million years ago.[1] The two chimpanzee lineages split about 2 million years ago. The rate of genetic change in the human lineage has been about 3% slower than the rate of genetic change in the chimpanzee lineages, and 11% slower than the rate of genetic change in the gorilla lineages, which split from the single undifferentiated human-chimpanzee lineage about 7 million years ago.[2]
If the same biological principles that are used to classify all other species except humans are applied to humans, then Homo sapiens and the two chimpanzee species, Pan troglodytes and Pan paniscus constitute a single biological genus. That is, either humans are a species of chimpanzee, Pan sapiens, or chimpanzees are species of humans, Homo troglodytes and Homo paniscus, or all three are species of a renamed genus such as Panhomo: Panhomo sapiens, Panhomo troglodytes and Panhomo paniscus. This similarly applies to all lineages that speciated after the split of the human and chimpanzee lineages. That is, to be ideationally consistent, the genus of all such species, including Australopithecus afarensis[3], is either Homo, Pan or an inclusive name like Panhomo.
Footnotes:
[1] Differences between species are difference by degree: every organism is related to every other organism through each generation to and from a common ancestor. Species only appear as different in kind because a synchronic perspective obscures the intervening steps between each organism, as Dawkins (2004), for example, points out.
[2] Elango, Thomas & Yi (2006).
[3] Interestingly, the oldest fossil puported to be human, Sahelanthropus tchadensis, is currently dated at 7 million years ago, which is before the proposed date of the human–chimpanzee split.