EVOLUTION AND FOSSIL HORSES
Equine Family Tree Much More
Expansive
By: John V. Wood
Conventional notion suggests horses simply became bigger over time, and changed from
diminutive shrub nibblers to the masterful grass-eating specimens we have today,
according to Bruce MacFadden, an equine forensic paleontologist at the University of
Florida. MacFadden has been working with horses since the 1970’s, and is considered a
world leader in equine understanding. He recently penned an article for the March issue
of the journal Science that suggested time wasn’t the only factor. The fossil horse study
suggested horses were adaptable creatures whose size, diet and range depended on
such factors as geography and climate as well.
“Modern day horses, zebras, asses and their kin are just a small part [of the Equine
family]. They are the tip of the iceberg in terms of us understanding what horses have
been like [throughout history],” said MacFadden. “The family tree of horses is much more
complicated than is portrayed in textbooks and museum exhibits today.”
MacFadden’s research has been going on for over a dozen years, and focuses on the
chemical makeup of fossilized equine teeth. By analyzing the carbon content of teeth,
MacFadden could determine a horse’s diet and movements, because some foods were
readily available and some were not. The size of the horse can also be determined,
because body size and tooth size are proportional.
MacFadden says scientists once thought the more primitive horses, which lived 55 million
to 20 million years ago, were primarily browsers that gave way the grazing in the late
Miocene period. This linear evolution might not be as straight as once thought.
MacFadden says there are quite a few “branches” to the equine family tree, because
some horses became mixed-feeders (eating both leaves and grasses) during this same
period. Also during this period, some horses grew larger while others became smaller or
remained the same size, he said.
“It was probably a combination of horses that were initially adapted to being forest-
dwellers, and then there is a whole new group of animals that became grazers during this
time. The primary diversity that is seen during this adaptive radiation is among the
grazers,” said MacFadden. “The grazing group gave rise to modern day horses.”
MacFadden also says this is not yet a widely accepted notion. “I would say, by specialists
in the field, people accept this theory. For people who are writing textbooks or people
interested in general science, it’s not widely accepted,” MacFadden said. “The more we
can let people know what the true story of horse evolution is, the better.”
