This is the type of display one typically sees in an old-fashioned Museum of Natural History:

Which implies that the tiny Hyracotherium gradually changed into a series of other species until the lineage evolved into the modern day horse, Equus.


The more modern account of horse evolution is depicted in this phylogenetic tree

indicating that ancestral lineages split into two descendant lineages at each clade (branch point). By definition, when an ancestral taxon splits into two new taxa, the ancestor becomes extinct, even if one of the new sister taxa is virtually identical to the original.

Using this model, it is incorrect to say that Equus evolved from Hyracotherium. One can say only that the two genera share a common ancestor, even though one of the two taxa is extinct.


For the same reason, it is incorrect to say that "humans evolved from apes" (or monkeys). They did NOT. But humans and apes and monkeys all share a common primate ancestor whose descendants diverged and gave rise to the various primate taxa alive today (and others that are now extinct).

One also cannot say that early hominid species gave rise to humans. The most we can say is that all hominids--fossil or extant--share a common ancestor.

The progression on the left (above) shows an anagenetic consideration of the fossil hominid skulls; the progression on the right shows a cladogenetic origin of the same skulls.

Below, a cladogram of Hominids...