Author: Kathleen Hunt (jespah@u.washington.edu) Title: Horse Evolution FAQ This is a compa
======================================================================
Author: Kathleen Hunt (jespah@u.washington.edu)
Title: Horse Evolution FAQ
======================================================================
This is a companion file for the transitional fossils FAQ. In this post I
will try to describe the modern view of evolution within the horse family.
I apologize in advance for the length; I didn't want to cut it down any more
than this, because horse evolution has been oversimplified too many times.
already. I wanted people to see some of the detail and complexity of the
fossil record of a fairly well known vertebrate group. (In fact, even at this
length, this post is still only a summary!) People who are in a hurry may
just want to read the intro and summary, and look at the tree.
********************************************************************
HORSE EVOLUTION FAQ (v4.1, Jan 4 1993)
OUTLINE:
1. Historical background -- why fossil horses are famous
2. Timescale and horse family tree
3. Small equids of the Eocene
4. Medium-sized browsing equids, late Eocene and Oligocene
5. The Miohippus radiation of browsing equids (24 My)
6. Horses move onto the plains: spring-foot & high-crowned teeth (18 My)
7. The merychippine radiation of the late Miocene (15 My)
8. One-toed grazing horses of the Pliocene & Pleistocene
9. Modern equines
10. Summary
11. References
1. 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.
2. TIMESCALE and HORSE FAMILY TREE
Recent 10,000 years ago to present
Pleistocene 2.5-0.01 My (million years ago)
Pliocene 5.3-2.5 My
Miocene 24-5.3 My
Oligocene 34-24 My
Eocene 54-34 My
And here's the tree...note that the timescale is a bit weird (e.g. the
Oligocene is compressed almost to nothing) to keep it from being too long.
All the names on the tree are genus names, so recall that each genus
encompasses a cluster of closely related species.
2My Old & New World Equus
\ | /
\ | /
4My Hippidion Equus Stylohipparion
| | Neohipparion Hipparion Cormohipparion
| | Astrohippus | | |
| | Pliohippus ---------------------------
12My Dinohippus Calippus \ | /
| | Pseudhipparion \ | /
| | | |
------------------------------------------- Sinohippus
15My \ | / |
\ | / Megahippus |
17My Merychippus | |
| Anchitherium Hypohippus
| | |
23My Parahippus Anchitherium Archeohippus
| | |
(Kalobatippus?)-----------------------------------------
25My \ | /
\ | /
|
35My |
Miohippus Mesohippus
| |
40My Mesohippus
|
|
|
45My Paleotherium |
| Epihippus
| |
Propalaeotherium | Haplohippus
| | |
50My Pachynolophus | Orohippus
| | |
| | |
------------------------------
\ | /
\ | /
55My Hyracotherium
3. SMALL EOCENE HORSES
The first equid was _Hyracotherium_, a small forest animal of the early
Eocene. This little animal (10-20" at the shoulder) looked nothing at all
like a horse. It had a "doggish" look with an arched back, short neck, short
snout, short legs, and long tail. It browsed on fruit and fairly soft foliage,
and probably scampered from thicket to thicket like a modern muntjac deer,
only stupider, slower, and not as agile. This famous little equid was once
known by the lovely name "Eohippus", meaning "dawn horse".
Some Hyracotherium traits to notice:
Legs were flexible and rotatable with all major bones present and unfused.
4 toes on each front foot, 3 on hind feet. Vestiges of 1st (& 2nd, behind)
toes still present. Hyracotherium walked on *pads*; its feet were like
a dog's padded feet, except with small "hoofies" on each toe instead of
claws.
Small brain with especially small frontal lobes.
Low-crowned teeth with 3 incisors, 1 canine, 4 distinct premolars and 3
"grinding" molars in each side of each jaw (this is the "primitive
mammalian formula" of teeth). The cusps of the molars were slightly
connected in low crests. Typical teeth of an omnivorous browser.
At this point in the early Eocene, equids were not yet very different from
the other perissodactyl groups; the _Hyracotherium_ genus includes some
species closely related to (or even ancestral to) rhinos and tapirs, as well
as species that are distinctly equine.
[Note: the particular species that probably gave rise to the rest of the
equids, _H. vassacciense_, may be renamed, perhaps to "Protorohippus".]
Though in retrospect we may consider Hyracotherium to be "primitive", it
was a very successful animal in its time, and seems to have found a nice
stable niche for itself. In fact, throughout most of the Eocene (a good long
20 million years), only minor evolutionary changes took place in
Hyracotherium and its near descendants. The body and feet stayed mostly
the same, with slight changes in the toes. The major change was in the
teeth; as Eocene equids started to eat more plant browse and less fruit,
they developed more grinding teeth to deal with the slightly tougher food.
_Orohippus_
In the early-middle Eocene (approx 50 My), there was a smooth, gradual
transition from Hyracotherium to a close relative, Orohippus (MacFadden,
1976). Overall, Orohippus looked much like Hyracotherium: 10-20" high at
the shoulder, still "doggish" with arched back, short legs, short neck, short
snout, and fairly small brain. Orohippus still had 4 toes on front and 3
behind, with hoofies, and was also "pad-footed". However, the vestiges of
the 1st and 2nd toes vanished.
The most significant change was in the teeth. The last premolar
changed in shape to become like a molar, giving Orohippus one more
"grinding tooth". Also, the crests on the teeth were more pronounced,
indicating Orohippus was eating tougher plant material.
_Epihippus_
Epihippus arose from Orohippus in the middle Eocene (approx. 47 My). Like
Orohippus and Hyracotherium, Epihippus was small, doggish, pad-footed, and
small-brained, with 4 toes in front and 3 behind. However, tooth evolution
was continuing. Now the last *two* premolars were like molars, giving
Epihippus *five* grinding cheek teeth. The crests on the cheek teeth were
well-formed, and still low-crowned.
There is a late form of Epihippus sometimes called _Duchesnehippus_. It's
unclear if this is a subgenus or a species of Epihippus. This animal was
basically an Epihippus with teeth similar to, but a bit more primitive than,
later Oligocene horses.
4. MEDIUM-SIZED BROWSING HORSES (Late Eocene & Oligocene)
As we move toward the Oligocene, horses start to change. The climate of
North America was becoming drier, and grasses were just evolving. The
vast forests were starting to shrink. The late Eocene horses responded by
developing tougher teeth and becoming a bit larger and leggier (for better
speed out in the open).
_Mesohippus_
The species _Mesohippus celer_ appears suddenly in the late Eocene, approx
40 My (such sudden speciations can occur when a population encounters new
selective forces and/or becomes isolated from the parent species. These
speciations are "sudden" only in geological terms, of course, where a few
million years is "sudden".) This animal was slightly larger than Epihippus,
24" at the shoulder. It didn't look as doggish, either. The back was less
arched, the legs a bit longer, the neck a bit longer, and the snout and face
distinctively longer. It had a shallow facial fossa, a depression on the
skull. (In later horses these fossae became complex, and handy for species
identification.) Mesohippus had three toes on its hind feet *and* on its
*front* feet -- the 4th front toe was reduced to a vestigial nubbin. As
before, Mesohippus was pad-footed.
Other significant changes:
Cerebral hemispheres notably larger -- has distinctly equine brain now.
Last *three* premolars are like the three molars, such that Mesohippus
(and all later horses) had a battery of *six* similar grinding "cheek
teeth", with one lonely little simple premolar in front.
Has same tooth crests as Epihippus, well-formed and sharp, more suitable
for grinding tougher vegetation.
_Miohippus_
Soon after _Mesohippus celer_ and its very close relative _Mesohippus
westoni_ appeared, a similar animal called _Miohippus assiniboiensis_
arose (approx. 36 My). This transition also occurred suddenly, but luckily a
few transitional fossils have been found that link the two genera. A typical
Miohippus was distinctly larger than a typical Mesohippus, with a slightly
longer skull. The facial fossa was deeper and more expanded. In addition,
the ankle joint had changed subtly.
Miohippus also began to show a variable extra crest on its upper cheek
teeth. In later horse species, this crest became a characteristic feature of
the teeth. This is an excellent example of how new traits originate as
variations in the ancestral population.
It was once thought that Mesohippus "transformed" gradually into Miohippus via
anagenetic evolution, so that only Miohippus continued. Recent evidence
shows that instead, Miohippus speciated (split off) from early Mesohippus
via cladogenetic evolution, and then Miohippus and Mesohippus
overlapped for some 4 million years. For instance, in one place in modern
Wyoming there were three species of late Mesohippus coexisting with two species
of Miohippus. (Prothero & Shubin, 1989)
5. THE MIOHIPPUS RADIATION (Early Miocene, 24 My)
Mesohippus finally died out in the mid-Oligocene. Miohippus continued for
a while as it was, and then, in early Miocene (24 My) began to speciate
fairly rapidly. The horse family began to split into at least 2 main lines of
evolution and one small side branch:
1) 3-toed browsers called "anchitheres". They were very successful, spread
into the Old World, and thrived for tens of millions of years. They retained
the small, simple teeth of Miohippus. Genera include _Anchitherium_ and
the large _Hypohippus_ and _Megahippus_.
2) a line of small "pygmy horses", e.g. _Archeohippus_. These horses did not
survive long.
3) a line that underwent a transformation from browsing to grazing, taking
advantage of the new grasses. Large grasslands were just beginning to
appear, thus creating a new ecological "opportunity" for grazers. Grass is
difficult to chew and wears down teeth rapidly (due to the silica in the
leaves) and thus a grass-eater needs tough teeth with ridges of some sort.
Open-country grass eaters, in addition, often benefit from being swift
runners with long legs. The evolution of this line of horses is described
below.
6.HORSES MOVE ONTO THE PLAINS: SPRING-FOOT & HIGH-CROWNED TEETH (Miocene, 18 My)
As this third line of Miocene horses began to specialize in eating grasses,
several changes occurred. First, the teeth changed to be better suited for
chewing harsh, abrasive grass. Small crests on the teeth enlarged and
connected together in a series of *ridges* for grinding. There was a
gradual increase in the *height of the tooth crowns*, so that the teeth
could grow out of the gum continuously as the tops were worn down
("hypsodont" teeth). And, in addition, the tooth crowns became harder due
to the development of a *cement* layer on the teeth.
Second, these horses started to become specialized runners. There
was a simultaneous increase in *body size, leg length, and length of the
face*. The bones of the legs began to *fuse* together, and the leg bones
and musculature became specialized for efficient forward-and-back
strides, with flexible leg rotation being eliminated. Most significantly, the
horses began to stand permanently on tiptoe (another adaptation for speed);
instead of walking on doglike pads, their weight was supported by *springy
ligaments* that ran under the fetlock to the big central toe.
All these changes occurred rapidly, and we are lucky to have a fairly
good fossil record during this time. This was one of the most interesting
times in horse evolution. The transitions in these characters are seen in:
_Kalobatippus_ -- this genus is not well known, but its teeth seem to be
intermediate between Miohippus and the later Parahippus (see below).
_Parahippus_ -- arose in early Miocene, 23 My.
A typical Parahippus was a little larger than Miohippus, with about
the same size brain and same body form. Parahippus was still three-toed,
and was just beginning to develop the springy ligaments under the foot.
Parahippus showed gradual and fluctuating changes in its teeth, including
the permanent establishment of the extra crest that was so variable in
Miohippus. In addition, various other cusps and crests were beginning to
join up in a series of *strong crests*, with slightly taller *tooth crowns*.
Parahippus evolved rapidly and was quickly transformed into a fully spring-
footed, hypsodont grazing horse called _Merychippus gunteri_. This burst of
evolution took place about 18-17 My. Later fossils of Parahippus (e.g. the
species _Parahippus leonensis_) are so similar to early Merychippus that
it's hard to decide where to draw the line between the genera.
_Merychippus_ 17 My (as in "Merry Kippus to all!" :-)
A typical Merychippus was about 10 hands (40") tall, the tallest equine yet.
The muzzle became elongated, the jaw became deeper, and the eye moved
farther back, to accommodate the large tooth roots. The brain was notably
larger, with a fissured neocortex and a larger cerebellum, making
Merychippus a smarter and more agile equine than the earlier horses.
Overall, Merychippus was distinctly recognizable as a horse, and had a
"horsey" head.
Merychippus was still 3-toed, but was fully spring-footed. This animal
stood permanently on tiptoe, supported and propelled by strong, springy
ligaments that ran under the fetlock. The side toes were still complete, but
began to be of varying sizes; some Merychippus species had full-size side
toes, while others developed small side toes that only touched the ground
during running. The central toe developed a large, convex, "horsey" hoof, and
the legs became longer. The radius and ulna of the forearm fused so that
leg rotation was eliminated. Likewise, the fibula of the shin was greatly
reduced. All these changes made Merychippus' legs specialized for just one
function: rapid running over hard ground.
Merychippus' teeth were fully high-crowned, with a thick layer of
cement, and with the same distinctive grazing tooth crests as Parahippus.
_Merychippus gunteri_ evolved into a slightly more advanced form,
_Merychippus primus_, in the middle/late Miocene.
7. THE MERYCHIPPINE RADIATION (Miocene, 15 My)
By the late Miocene, Merychippus was the one of the first bona-fide speedy
plains grazers. (Simpson, 1961, called Merychippus "the horse with a new
look"). Merychippus underwent rapid speciation, and gave rise to at least 19
new grazing horse species in three major groups. This explosive burst of
horse evolution is often called the "merychippine radiation". The three
major groups were:
1) Three-toed grazers known as "hipparions". These were tremendously
successful and split into 4 genera and at least 16 species, eventually
covering a variety of niches for small and large grazers and browsers. They
developed large and elaborate facial fossae. Hipparions spread from the New
World into the Old World in several waves of migration.
2) A line of smaller horses including _Protohippus_ and _Calippus_,
collectively called "protohippines".
3) A line of "true equines" in which the side toes sometimes began to
decrease in size. In this flurry of evolution, Merychippus primus gave
rise to two later merychippines called M. sejunctus and M. isonesus, who had
a mixture of "primitive" (Parahippus-like), hipparion, and equine features.
They, in turn, gave rise to M. intermontanus, which begat M. stylodontus and
M. carrizoensis. These last two looked quite "horsey" and gave rise to a
set of larger three-toed and one-toed horses known as the "true equines"
(see below). Crystal clear, right?
As this brief list shows, new species arose in rapid succession in all three
of these groups. This rapid speciation makes it hard to determine exactly
which species arose from exactly which others.
About 10 My, the horse family reached an apex of diversity (of species and
of genera) and sheer numbers which it has never equalled since. The Old
and New Worlds both seemed overrun with a wide variety of hipparions,
protohippines, and "true equines", large and small, forest browsers and
plains grazers. Throughout the evolution of all these related
merychippine descendents, the facial fossae got deeper and more elaborate.
With so many equine species overlapping at once, these facial fossae may
have housed species-specific glands of some sort, similar to the scent-
marking glands of modern antelopes and deer.
8. ONE-TOED HORSES (Late Miocene, Pliocene & Pleistocene)
Let's leave the hipparions and protohippines now, and concentrate on
the merychippine line that led to the "true equines". The late
merychippine species of this line, such as M. carrizoensis, were large
horses with small side toes. They gave rise to at least 2 separate
groups of horses that independently lost their side toes. This occurred as
*side ligaments* developed around the fetlock to help stabilize the central
toe during running. These one-toed horses include:
_Pliohippus_ -- arose in middle Miocene (~15 My) as a three-toed
horse. Gradual loss of the side toes is seen in Pliohippus through 3
successive strata of the early Pliocene. Pliohippus was very similar to
Equus and until recently was thought to be the direct ancestor of Equus,
except for two significant differences. First, Pliohippus's skull has deep
facial fossae, whereas Equus has no facial fossae at all. Second,
Pliohippus's teeth are strongly curved, and Equus's teeth are very straight.
Though Pliohippus is obviously related to Equus, it probably didn't give rise
to Equus.
_Astrohippus_ (~10My) was another one-toed horse that arose shortly
after Pliohippus. Astrohippus also had large facial fossae, and was
probably a descendent of Pliohippus.
Finally, a third one-toed horse called _Dinohippus_ (recently
discovered) arose about 12 My. The exact ancestor of Dinohippus is not yet
known (see Evander, 1989). The earliest known species are D. spectans, D.
interpolatus, and D. leidyanus. They look smashingly like Equus in foot
morphology, teeth, and skull. The teeth were slightly *straighter* than
Merychippus, and the facial fossae were significantly *decreased*. A
slightly later species was D. mexicanus, that showed even straighter teeth
and even smaller fossae. Dinohippus was the most common horse in North
America in the late Pliocene, and almost certainly gave rise to Equus.
(Recall that Equus has very straight teeth and no fossae.)
The Isthmus of Panama arose at this point. Some very early Dinohippus
species gave rise to the "hippidions", stocky, short-legged, one-toed
horses with odd boxy skulls (~4 My). They travelled into the South America
and thrived there briefly.
Throughout the end of the Pliocene, Dinohippus showed a gradual
decrease in the facial fossae, straightening of the teeth, and other gradual
changes, as Dinohippus smoothly graded into Equus. (Hulbert, 1989)
_Equus_ -- arose in late Pliocene about 4 My.
Finally we arrive at Equus, the genus of all modern equines. The first
Equus were 13.2 hands tall (pony size), with a classic "horsey" body -- rigid
spine, long neck, long legs, fused leg bones with no rotation, long nose,
flexible muzzle, deep jaw. The brain was a bit larger than in early
Dinohippus. Like Dinohippus, Equus was (and is) one-toed, with side ligaments
that prevent twisting of the hoof, and has high-crowned, straight grazing
teeth with strong crests lined with cement.
Members of Equus still retain the genes for making side toes. Usually
these express themselves only as the vestigial "splint bones" of toes 2 and
4, around the large central 3rd toe. Very rarely, a modern Equus is born
with small but fully-formed side toes. (see Gould, "Hen's Teeth and Horses'
Toes".)
The earliest known Equus species were a set of three "simple Equus"
species collectively known as the _Equus simplicidens_ group. They still had
some primitive traits from Dinohippus, including a slight facial fossa. They
had zebra-like bodies (relatively stocky with a straight shoulder
and thick neck), and short, narrow, donkey-like skulls. They probably had
stiff, upright manes, ropy tails, medium-sized ears, striped legs, and
at least some striping on the back (all traits shared by modern equines).
They quickly diversified into at least 12 new species in 4 different groups,
in a burst of evolution reminiscent of the great merychippine radiation.
All these Equus species coexisted with other one-toed horses (such as
_Astrohippus_) and with various successful hipparions and protohippines,
which had been merrily evolving on their own paths.
During the first major glaciations of the late Pliocene (2.6 Ma), certain
_Equus_ species crossed to the Old World. Some entered Africa and
diversified into the modern zebras. Others spread across Asia, the Mideast, &
N. Africa as desert-adapted onagers and asses. Still others spread across
Asia, the Mideast, and Europe as the true horse, _E. caballus_. Other
_Equus_ species spread into South America. The Equus genus was perhaps
the most successful perissodactyl genus that ever lived -- even before
domestication by humans.
Compare Equus to Hyracotherium and see how much it has changed. In
no way can Equus and Hyracotherium be considered the same "kind". The
change from Hyracotherium to Equus is truly long-term, large-scale
evolution.
9. MODERN EQUINES (Recent)
The three-toed horses gradually died out, perhaps outcompeted by the
phenomenally successful artiodactyls (or not). Most of the one-toed horses
in North America also died out, as the Ice Ages started. (The causes of
these extinctions are unknown.) However, one-toed Equus was very successful.
Until about 1 million years ago, there were Equus species all over Africa,
Asia, Europe, North America, and South America, in enormous migrating herds
that must easily have equalled the great North American bison herds, or the
huge wildebeest migrations in Africa.
In the late Pleistocene there was a set of devastating extinctions that
killed off most of the large mammals in North and South America. All the
horses of North and South America died out (along with the mammoths and
saber-tooth tigers). These extinctions seem to have been caused by a
combination of climatic changes and overhunting by humans, who had just
reached the New World. For the first time in tens of millions of years, there
were no equids in the Americas.
The only members of Equus -- and of the entire family Equidae -- that
survived to historic times were:
Order Perissodactyla
Family Equidae
Genus Equus
_Equus burchelli_ -- the Plains zebra of Africa, including "Grant's
zebra", "Burchell's zebra", "Chapman's zebra", the half-striped
Quagga, and other subspecies. The Plains zebra is what people
usually think of as the "typical zebra", with rather wide vertical
stripes, and thick horizontal stripes on the rump.
_Equus zebra_ -- the Mountain zebra of South Africa. This is the little
zebra with the dewlap and the gridiron pattern on its rump.
_Equus grevyi_ -- Grevy's zebra, the most horse-like zebra. This is
the big zebra with the very narrow vertical stripes and huge ears.
_Equus caballus_, the true horse, which once had several subspecies.
_Equus hemionus_ -- the desert-adapted onagers of Asia & the Mideast,
including the kiang (formerly E. kiang).
_Equus asinus_ -- the true asses & donkeys of northern Africa. (The
African wild asses are sometimes called E. africanus.)
[I have a separate file about the relationships & current status of all
surviving wild equines, including information about captive breeding programs.
E-mail for details.]
10. SUMMARY
For many people, the horse family remains the classic example of evolution.
As more and more horse fossils have been found, some ideas about horse
evolution have changed, but the horse family remains a good example of
evolution. In fact, we now have enough fossils of enough species in enough
genera to examine subtle details of evolutionary change, such as modes of
speciation.
In addition to showing that evolution has occurred, the fossil Equidae also
show the following characteristics of evolution:
1. Evolution does not occur in a straight line toward a goal, like a ladder;
rather, evolution is like a branching bush, with no predetermined goal.
Horse species were constantly branching off the "evolutionary tree"
and evolving along various unrelated routes. There's no discernable
"straight line" of horse evolution. Many horse species were usually present
at the same time, with various numbers of toes, adapted to various
different diets. In other words, horse evolution had no inherent direction.
We only have the impression of straight-line evolution because only one
genus happens to still be alive, which deceives some people into thinking that
that one genus was somehow the "target" of all the evolution. Instead, that
one genus is merely the last surviving branch of a once mighty and sprawling
"bush".
The view of equine evolution as a complex bush with many
contemporary species has been around for several decades, and is commonly
recounted in modern biology and evolution textbooks.
2. There are no truly consistent "trends".
Tracing a line of descent from Hyracotherium to Equus reveals several
apparant trends: reduction of toe number, increase in size of cheek teeth,
lengthening of the face, increase in body size. But these trends are not seen
in all of the horse lines. On the whole, horses got larger, but some horses
(Archeohippus, Calippus) then got smaller again. Many recent horses
evolved complex facial pits, and then some of their descendants lost them
again. Most of the recent (5-10 My) horses were three-toed, not one-toed,
and we see a "trend" to one toe only because all the three-toed lines have
recently become extinct.
Additionally, these traits do not necessarily evolve together, or at a
steady rate. The various morphological characters each evolved in fits and
starts, and did *not* evolve as a suite of characters. For example,
throughout the Eocene, the feet changed little, and only the teeth evolved.
Throughout the Miocene, both feet and teeth evolved rapidly. Rates of
evolution depend on the ecological pressures facing the species.
The "direction" of evolution depends on the ecological challenges
facing the individuals of a species and on the variation in that species, not
on an inherent "evolutionary trend".
3. New species can arise through several different evolutionary mechanisms.
Sometimes, new species split off suddenly from their ancestors
(e.g., Miohippus from Mesohippus) and then co-existed with those ancestors.
Other species came into being through anagenetic transformation of the
ancestor, until the ancestor had changed appearance enough to be given a
new name (e.g. Equus from Dinohippus). Sometimes only one or a few
species arose; sometimes there were long periods of stasis (e.g.
Hyracotherium throughout the early Eocene); and sometimes there were
enormous bursts of evolution, when new ecological opportunities arose (the
merychippine radiation). Again, evolution proceeds according to the
ecological pressures facing the individuals of a species and on the variation
present within that species. Evolution takes place in the real world, with
diverse rates and modes, and cannot be reduced to a single, simple process.
A Question for Creationists:
Creationists who wish to deny the evidence of horse evolution should
careful consider this: *how else can you explain the sequence of horse
fossils?* Even if creationists insist on ignoring the transitional fossils
(many of which *have* been found), again, how can the unmistakable
SEQUENCE of these fossils be explained? Did God create Hyracotherium,
then kill off Hyracotherium and create some Hyracotherium-Orohippus
intermediates, then kill off the intermediates and create Orohippus, then
kill off Orohippus and create Epihippus, then allow Epihippus to
"microevolve" into Duchesnehippus, then kill off Duchesnehippus and create
Mesohippus, then create some Mesohippus-Miohippus intermediates, then
create Miohippus, then kill off Mesohippus, etc.....each species
coincidentally similar to the species that came just before and came just
after?
Creationism utterly fails to explain the sequence of known horse
fossils from the last 50 million years. That is, without invoking the "God
Created Everything To *Look* Just Like Evolution Happened" Theory.
[And I'm not even mentioning all the *other* evidence for evolution that
is totally independent of the fossil record -- developmental biology,
comparative DNA & protein studies, morphological analyses, biogeography, etc.
The fossil record, horses included, is only a small part of the story.]
Truly persistent and/or desperate creationists are thus forced into
illogical, unjustified attacks of fossil dating methods, or irrelevant
and usually flat-out wrong proclamations about a supposed "lack" of
"transitional forms". It's sad. To me, the horse fossils tell a
magnificent and fascinating story, of millions of animals living out
their lives, in their natural world, through millions of years. I am a
dedicated horse rider and am very happy that the one-toed grazing Equus
survived to the present. Evolution in no way impedes my ability to admire
the beauty and nobility of these animals. Instead, it enriches my
appreciation and understanding of modern horses and their rich history.
*****************************************************************
"All the morphological changes in the history of the Equidae can
be accounted for by the neo-Darwinian theory of microevolution:
genetic variation, natural selection, genetic drift, and speciation."
(Futuyma 1986, p.409)
"Because its complications are usually ignored by biology textbooks,
creationists have claimed the horse story is no longer valid. However, the
main features of the story have in fact stood the test of time...."
(Futuyma 1982, p. 85)
"When asked to provide evidence of long-term evolution, most scientists
turn to the fossil record. Within this context, fossil horses are among the
most frequently cited examples of evolution. The prominent Finnish
paleontologist Bjorn Kurten wrote: 'One's mind inevitably turns to that
inexhaustible textbook example, the horse sequence. This has been cited --
incorrectly more often than not -- as evidence for practically every
evolutionary principle that has ever been coined.' This cautionary note
notwithstanding, fossil horses do indeed provide compelling evidence in
support of evolutionary theory."
(MacFadden 1988, p. 131)
"The fossil record [of horses] provides a lucid story of descent with change
for nearly 50 million years, and we know much about the ancestors of
modern horses."
(Evander 1989, p. 125)
"It is evolution that gives rhyme and reason to the story of the horse family
as it exists today and as it existed in the past. Our own existence has the
same rhyme and reason, and so has the existence of every other living
organism. One of the main points of interest in the horse family is that it
so clearly demonstrates this tremendously important fact."
(Simpson, 1961, p. xxxiii)
******************************************************************
REFERENCES
I've tried to incorporate all the recent research I could find into this post.
For more information, non-scientists may want to start with Simpson's
1961 book, _Horses_. This book is a classic, readable account of horse
evolution, and though it's now somewhat outdated, I think it's still the most
accessible introduction to the topic. However, I *strongly* recommend that
Simpson's book be supplemented with newer information from MacFadden's nice
summary (1988) and/or Prothero & Schoch's _The Evolution of Perissodactyls
(1989). These and other selected references are listed below.
Thanks to Larry Moran for the prototype of the ASCII horse tree and
other various notes.
Bennett, D.K. 1986? (year not on my xerox! argh.) The origins of breeds.
Equus 110:33, 11:37, 112:37.
This is a three-part series in a good-quality trade magazine, written for
horse owners who have some interest in science and evolution. (Further
references are in the articles.) The author is a vertebrate paleontologist
who specializes in the evolution, form, and function of modern Equus.
Her analysis shows that E. caballus had at least 5 subspecies before
domestication.
Colbert, E.H. 1980. _Evolution of the Vertebrates_, 3rd edition. John Wiley
& Sons, New York.
Carroll, R.L. 1988. _Vertebrate Paleontology and Evolution_. WH Freeman
& Co., New York.
(These are two standard texts on vertebrate fossils & evolution. Colbert
has a 4th edition out now.)
Futuyma, D.J. 1982. _Science on Trial: The Case for Evolution. Pantheon
Books, New York.
(A well-written book on the evidence for evolution, written for the layperson.)
Futuyma, D.J. 1986. _Evolutionary Biology_. Sinauer Associates,
Sunderland, Mass.
(A standard text covering theories of *how* evolution occurs -- doesn't
stress evidence for evolution per se.)
Gould, S.J. (year?) _Hen's Teeth And Horse's Toes_.
Gould, S.J. (year?) _Bully for Brontosaurus_.
(Collections of essays written for _Natural History_ magazine. "Hen's
Teeth..." has essays on horse side toes and zebra stripes; "Bully..." contains
essays on "fox-terrier size" Hyracotherium and on the fallacy of perceiving
a direction of evolution in the horse family. Other essays are interesting
too. Sorry I don't have more precise references handy....my copy of Hen's
Teeth is in Boston, and Bully for B is at home.)
Hildebrand, M. 1987. The mechanics of horse legs. Amer. Sci. 75:594-601.
(not about evolution, but interesting & useful nonetheless.)
Janis, C. 1976. The evolutionary strategy of the Equidae and the origins of
rumen and cecal digestion. Evolution 30:757-774.
(An interesting analysis of the significance of hindgut fermentation in
equids, and on why the Equidae tend not to have high species diversity.)
Lowenstein, J.M., and O.A. Ryder. 1985. Immunological systematics of the
extinct quagga (Equidae). Experientia 41:1192-1193.
(The authors studied molecules from skins of the extinct quagga, and
conclude it was a subspecies of the plains zebra.)
MacFadden, B.J. 1976. Cladistic analysis of primitive equids with notes on
other perissodactyls. Syst. Zool. 25(1):1-14.
(An analysis of the interrelationships of Hyracotherium, Orohippus,
Epihippus, the paleotheres, and other early perissodactyls.)
MacFadden, B.J. 1984. Systematics and phylogeny of the _Hipparion_,
_Neohipparion_, _Nannippus_, and _Cormohipparion_ (Mammalia, Equidae)
from the Miocene and Pliocene of the New World. Bull. Am. Mus. Nat. Hist.
179:1-196.
(*Extremely* detailed analysis of evolution and interrelationships of the
hipparions. [Okay, okay, I didn't read the whole thing.] Relies to a
large extent on the distinctive morphology of the facial fossae in
different species.)
MacFadden, B.J. 1984. _Astrohippus_ and _Dinohippus_ from the
Yepomera local fauna (Hemphillian, Mexico) and implications for
the phylogeny of one-toed horses. J. Vert. Paleon. 4(2):273-283.
(Description & discussion of Pliohippus, Astrohippus, and Dinohippus.
Concludes that Dinohippus was probably the ancestor of Equus, and Pliohippus
was probably the ancestor of Astrohippus.)
MacFadden, B.J. 1985. Patterns of phylogeny and rates of evolution
in fossil horses: hipparions from the Miocene and Pliocene of
North America. Paleobiology 1(3):245-257.
Analyzes the evolution of hipparion species. Of the 16 known species, 6
appear too suddenly for the mode of speciation to be determined. Of the 10
that appeared gradually enough for speciation mode to be determined, 5 have
originated by anagenesis (transformation of an ancestor species into a
descendent species, such that the ancestor "disappears") and 5 by
cladogenesis (splitting off of a new species from an ongoing ancestor
species, such that the 2 species continue to exist together.)
MacFadden, B.J. 1986. Late Hemphillian monodactyl horses (Mammalia,
Equidae) from the Bone Valley formation of central Florida. J. Paleontology
60(2):466-475.
(Description of two recent discovered advanced one-toed horse species:
_Astrohippus stocki_ and _Dinohippus mexicanus_.)
MacFadden, B.J. 1988. Horses, the fossil record, and evolution: a current
perspective. Evol. Biol. 22:131-158.
(A useful and readable update on current evidence & theories of horse
evolution.)
MacFadden, B.J. 1993. [a new book about horse evolution. I have not read
it yet but am trying to get a copy. Over $70! sheesh]
MacFadden, B.J., J.D. Bryant, and P.A. Mueller. 1991. Sr-isotopic,
paleomagnetic, and biostratigraphic evidence of horse evolution:
evidence from the Miocene of Florida. Geology 19:242-245.
This is an interesting example of the variety of dating methods
paleontologists use to date their finds. MacFadden et al. dated the
Parahippus --> Merychippus transition at a Florida site with paleomagnetic
data and Sr/Sr dates, and also by cross-correlation to other sites dated
with Sr/Sr, K/Ar, Ar/Ar, zircon fission-track, and paleomagnetic dating
methods. Surprise, surprise, all the dates were consistent at roughly 16 My.
MacFadden, B.J., & R.C. Hubbert. 1988. Explosive speciation at the base of
the adaptive radiation of Miocene grazing horses. Nature 336:466-468.
(An interesting summary of the merychippine radiation. Has a nice horse
tree, too. MacFadden's horse tree is used by almost everyone these days.)
MacFadden, B.J., & M.F. Skinner. 1981. Earliest holarctic hipparion,
_Cormohipparion goorisi_ n.sp. (Mammalia, Equidae) from the Barstovian
(medial Miocene) Texas gulf coastal plain. J. Paleontology 55(3):619-627.
(Description of a hipparion that was found to have crossed into the Old World
from the New World sooner than previously realized.)
Prothero, D.R., & R.M. Schoch, eds. 1989. _The Evolution of Perissodactyls_.
Clarendon Press, New York.
A compilation of current research and theories of perissodactyl evolution.
The following chapters were particularly useful:
Evander, R.L. Phylogeny of the family Equidae. pp. 109-126
MacFadden, B.J. Dental character variation in paleopopulations and
morphospecies of fossil horses and extant analogs. pp. 128-141
Hulbert, R.C. Phylogenetic interrelationsihps and evolution of North
American late Neogene Equinae. pp. 176-196.
Prothero, D.R., & R.M. Schoch. Origin and evolution of the perissodactyla:
summary and synthesis. pp. 504-529.
Prothero, D.R., & N. Shubin. The evolution of Oligocene horses. pp.142-175.
Winans, M.C. A quantitative study of North American fossil species of
the genus _Equus_. pp. 262-297.
Radinsky, L. 1983. Allometry and reorganization in horse skull proportions.
Science 221 (16 Sept):1189-1191
(Analysis of horse skull changes around the time that horses developed
high-crowned teeth, between 15 and 25 million years ago.)
Renders, E. 1984. The gait of _Hipparion_ sp. from fossil footprints in
Laetoli, Tanzania. Nature 308:179-181.
(Interesting paper describing fossil hoofprints of an adult female
hipparion and her foal. They were using a gait called a "running walk".)
Simpson, G.G. 1961. _Horses_. Doubleday & Co., New York.
(An interesting and readable, though outdated, account of horse evolution.
Written for the intelligent non-scientist by a prominent paleontologist.)
Thomason, J.J. 1986. The functional morphology of the manus in the
tridactyl equids _Merychippus_ and _Mesohippus_: paleontological inferences
from neontological models. J. Vert. Pal. 6(2):143-161.
(Analysis of the pad-foot to spring-foot transition.)
*******************************************************************
"[Fossils] are animals, just as full of life as you are, even though they
occur at different points in the endless stream of time. Within their own
segments of this stream, they breathe, eat, drink, breed, fight, and live
their own lives..."
(Simpson, 1961, p. xxxiv)
don't know yet if there's anything substantially new in there.
E-Mail Fredric L. Rice / The Skeptic Tank