Evolution of the Horse
By Kathleen Hunt
Historical Background
In the 1870's, the paleontologist O.C. Marsh published a description of
newly discovered horse fossils from North America. At the time, very few
transitional fossils were known, apart from Archeopteryx.
The sequence of
horse fossils that Marsh described (and that T.H. Huxley popularized) was a
striking example of evolution taking place in a single lineage. Here, one
could see the fossil species "Eohippus" transformed into an almost totally
different-looking (and very familiar) descendent, Equus, through a series of
clear intermediates.
Biologists and interested laypeople were justifiably
excited. Some years later, the American Museum of Natural History assembled
a famous exhibit of these fossil horses, designed to show gradual evolution
from "Eohippus" (now called Hyracotherium) to modern Equus.
Such exhibits
focussed attention on the horse family not only as evidence for evolution
per se, but also specifically as a model of gradual, straight-line
evolution, with Equus being the "goal" of equine evolution. This story of
the horse family was soon included in all biology textbooks.
As new fossils were discovered, though, it became clear that the old model
of horse evolution was a serious oversimplification. The ancestors of the
modern horse were roughly what that series showed, and were clear evidence
that evolution had occurred. But it was misleading to portray horse
evolution in that smooth straight line, for two reasons:
1. First, horse evolution didn't proceed in a straight line. We now know
of many other branches of horse evolution. Our familiar Equus is merely one
twig on a once-flourishing bush of equine species. We only have the illusion
of straight-line evolution because Equus is the only twig that survived.
(See Gould's essay "Life's Little Joke" in Bully for Brontosaurus for more
on this topic.)
2. Second, horse evolution was not smooth and gradual. Different traits
evolved at different rates, didn't always evolve together, and occasionally
reversed "direction". Also, horse species did not always come into being by
gradual transformation ("anagenesis") of their ancestors; instead, sometimes
new species "split off" from ancestors ("cladogenesis") and then co-existed
with those ancestors for some time. Some species arose gradually, others
suddenly.
Overall, the horse family demonstrates the diversity of evolutionary
mechanisms, and it would be misleading -- and would be a real pity -- to
reduce it to an oversimplified straight-line diagram.
With this in mind, I'll take you through a tour of the major genera of the
horse family, Equidae. CAUTION: I will place emphasis on those genera that
led to the modern Equus. Do not be misled into thinking that Equus was the
target of evolution! Bear in mind that there are other major branches of the
horse tree that I will mention only in passing. (See the horse tree for a
lovely ASCII depiction.)
Small preface: All equids (members of the family Equidae) are
perissodactyls -- members of the order of hoofed animals that bear their
weight on the central 3rd toe. (Other perissodactyls are tapirs and rhinos,
and possibly hyraxes.) The most modern equids (descendents of Parahippus)
are called "equines". Strictly speaking, only the very modern genus Equus
contains "horses", but I will call all equids "horses" rather
indiscriminately.
Most horse species, including all the ancestors of Equus, arose in North
America.
(continued)
Judy
http://icehorses.net
http://clickryder.com
