* HORSE EVOLUTION FAQ * OUTLINE: 1. Historical background why fossil horses are famous 2.
FROM: Kathleen Hunt jespah@milton.u.washington.edu
********************* HORSE EVOLUTION FAQ *************************
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.
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.)
Tiny 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.
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 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. 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"). 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".] Hyracotherium was a successful animal 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 become 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 similar descendant _Mesohippus
westoni_ appeared, a similar animal called _Miohippus
assiniboiensis_ arose (approx. 36 My). This transition also
occurred suddenly, but luckily a few transitional fossil 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" into
Miohippus via anagenetic evolution, so that only Miohippus
continued. Recent evidence shows that instead, Miohippus speciated
from early Mesohippus (cladogenetic evolution), and then Miohippus
and later Mesohippus overlapped for some 4 million years. In one
locale in modern Wyoming there were three species of late Mesohippus
and two 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. (Thomason, 1988)
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 springfooted, 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
A typical Merychippus was about 10 hh (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, _M. 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 into the Old World from the New 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 the middle Miocene, Merychippus primus gave
rise to M. sejunctus and then to M. isonesus. These two later
merychippines had a mixture of "primitive" (Parahippus-like),
hipparion, and equine features, and were probably ancestral to the
true equines. Specifically, they probably gave rise to M.
intermontanus, which gave rise to M. stylodontus and M.
carrizoensis. These last two gave rise to a set of larger three-toed
and one-toed horses (see below).
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. The
horse family reached an apex of diversity and numbers about 10 My,
when the Old World and New World seemed overrun with a wide
diversity of hipparions, protohippines, and "true equines".
Throughout the evolution of these closely 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.
8. ONE-TOED HORSES (Late Miocene, Pliocene & Pleistocene)
The late merychippine species (such as M. carrizoensis) were large
horses with small side toes, and 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. The Isthmus
of Panama arose at this point. Some very early Dinohippus 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 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 called the _E. simplicidens_ group. These retained some
primitive traits from Dinohippus, including a slight facial fossa.
These early "simple Equus" 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 wild Equus). They quickly diversified into at least 12 new
species in 4 different groups. 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, 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, 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. All the horses of South and
North America perished in the late Pleistocene extinctions (the ones
that also wiped out the sabertooths & mammoths). The only members of
Equus -- and of the entire family Equidae -- that survived into
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. _Equus zebra_ -- the Mountain
zebra of South Africa. _Equus grevyi_ -- Grevy's zebra, the most
horse-like zebra. _Equus caballus_, the true horse, which once had
several subspecies. _Equus hemionus_ -- the desert-adapted onagers
of Asia & the Mideast. _Equus asinus_ -- the true asses & donkeys of
northern Africa
Of the three zebra species, only the Plains zebra still thrives,
though the quagga went extinct in 1883 and other subspecies are
dwindling. The Mountain zebra is threatened and the Grevy's zebra is
endangered. The onagers and wild asses are endangered, though the
domestic donkey is thriving. _E. caballus_, the true horse, is
thought to have had 5 major subspecies in Europe and Asia. Three
were domesticated and have not been seen in a true wild state for
thousands of years (they correspond roughly to the Arabian, the
draft horse/pony, and the warmblood). A fourth, the tarpan, survived
in Poland till 1900 but then went extinct. The fifth, the
Przewalski's horse, went extinct in the wild and for decades barely
survived in zoos. Some P's horses have recently been reintroduced to
the wild. (Many people consider the Przewalski's horse to be a
separate species, but it is fully capable of intrbreeding with
domestic E. caballus, to the distress of the zookeepers.)
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".
4. 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.
REFERENCES
There has been much recent research on perissodactyls, both studying
new fossils and re-evaluating old ones. I've tried to incorporate
all of the relevant recent work I could find into this post. For
more information, non-scientists may want to look up 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 1988 summary, and/or Prothero & Schoch's _The
Evolution of Perissodactyls_ (1989). These and other selected
references are listed below.
I was originally planning to include a summary of recent research in
this post, but the post is so long that instead I'll make that into
a separate file. I'll e-mail it to anyone who's interested.
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 horse 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. A nicely done
analysis of the origins of _E. caballus_.)
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 isn't in paperback yet!)
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.)
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. 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., 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.)
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
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.
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.
(An analysis of the pad-foot to spring-foot transition.)
"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)
"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)
"[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)
E-Mail Fredric L. Rice / The Skeptic Tank