Zebra evolution!

Zebras are very closely related to the modern horse. We can trace “horse-like” animals back at least 50 million years. First, a short chart to help you get an idea of how long ago we’re talking about . . .

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

The first sets of fossils that have similarities to the modern horse are of the genus Hyracotherium. These animals lived in the Eocene period (we know this because of the age of the rocks where we found the fossils!), and we’ve found fossils of Hyracotherium in numerous sites in the United States and in Europe. Based on the skeletons, we think it looked something like this . . .

Hyracotherium had 4 toes on the front feet, and 3 on the back feet. Seems kind of nitpicky, but the structure of the foot bones actually helps us place this animal in its rightful ancestry!

We know Hyracotherium was an herbivore because of the shape of its teeth. Animals that eat mainly plants have large, flat back teeth to help them grind down the cell walls in the plants. Hyracotherium probably ate easy things like fruit instead of more difficult things like grass, because its teeth are very flat, with no pointed crests for friction. At this point, America and Europe were covered with lots of forests, so there was plenty of soft fruit to go around.

Hyracotherium must have been very well suited to its environment, because we can find almost 20 million years worth of Hyracotherium fossils, with very little change from the earliest specimen to the most recent.

Now, on to Orohippus. Orohippus fossils are found only in the western United States, and they date back to about 2 million years after the first Hyracotherium fossils in that area. Notice that it’s possible for two species to co-exist at the same time! Most likely, Orohippus came from a group of Hyracotherium that lived in the Wyoming and Oregon area. Small mutations in their DNA over time caused that population to change slightly. Orohippus’s teeth looked a little different from Hyracotherium’s. There was more grass than fruit in this area, so these little guys had molars with larger grinding edges so that they could chew the grass more thoroughly. Also, Orohippus had an extra molar, and lost what are known as vestigal toes.

Within another 3 million years (we’re up to 47 million years ago), the Orohippus population apparently changed into a completely new species, called Epihippus. (If you took an Epihippus and an Orohippus and tried to mate them, they probably could not have had kids together.) As the terrain in the western U.S. changed to even more grass and even less fruit, animals with more grinding teeth would have had an advantage. Epihippus has a total of 5 grinding teeth on each side.

Now, let’s go forward to about 40 million years ago. A new species appears quite suddenly in the fossil record. It’s bigger than its ancestors; the face, legs, and neck are all longer; the brain space in the skull in bigger; it’s lost the fourth toe on its front feet; and it has even more grinding molars. This is the Mesohippus. It was well-suited to grazing grass and leaves, and to running away from predators on the hard ground. (More toes are useful in boggy areas, but as the terrain changed over time, animals with fewer toes were better at running on the harder ground.)

Mesohippus looks more like the Orohippus than the Epihippus, so we think that Orohippuses are the ancestor of Mesohippuses. As far as we can tell, Epihippuses lived happily for a few million years, but then died out. It’s always possible that we’ll find some fossils that look similar to Epihippuses, however, so stay tuned!

We won’t go into the details of the next several million years. The chart gives you an idea of how many other equine species we’ve found evidence of. Not all of the species evolved into something else, as you can see. Evolution does not usually happen in a straight line!

A few trends during this time in horse/zebra evolution:

·  Teeth better suited to grinding up grass. Horses’ teeth continually grow out of their gums because the silica in grass really wears down the teeth. This adaptation showed up during the Miocene era. Also, the teeth in these species have a cement layer on top.

·  These animals became very good runners. Early equines had feet with pads (kind of like a dog or cat). Now, though, they run on tiptoe. Better for speed! And it seems that one tiptoe is even better than three tiptoes, since that’s what we see in the fossil record as time went by. The shape of the bones and muscles became better at forward-and-back motion, at the expense of leg rotation. (The muscles don’t survive as fossils, but we can tell what they looked like by the points on the bones where they attached. Cool, huh? J)

·  Bigger! These guys were living on the plains, where there’s nowhere to hide when the saber-toothed tiger comes along. The bigger you are, the faster you can run. (Up to a point, of course. This is why we don’t see a large population of giant horses – any horse that was too big to run fast would have quickly become a tiger snack.)

·  Facial fossae (dips) in the skull. We think these spaces held scent glands that allowed members of the same species to recognize each other. Notice all of the species that were living at the same time during the Miocene period. If you are a Hypohippus, and you don’t recognize the scent of a Archaeohippus (or, at least, don’t find it attractive), you’re not going to mate with that Archaeohippus. Your genes will stay separate from that other species, driving the evolution of both populations.

Finally, about 4 million years ago, we find the first fossils that look almost completely like modern horses. During one of the ice ages around 2.6 million years ago, members of the Equus genus crossed over into Europe, and spread into Asia and Africa. Over time, each population adapted to the habitats they found themselves in. By 1 million years ago, there were numerous Equus species all over the world (except Australia). However, climate change and hunting by early humans wiped out most of the species. Now, there are four Equus species in Africa:

Equus burchelli (the one we’re interested in), Equus grevyi, Equus zebra, and Equus asinus. (There used to be a fifth, Equus quagga, but the Europeans hunted and ate all of them in the 1800s.)

All of these influences helped make the zebra what it is today! I’ll never quiz you on the individual equine species, but there are a few important points to take away from this zebra example:

·  Evolution isn’t “trying” to produce a specific animal. If the climate and terrain of the western U.S. hadn’t changed 40 million years ago, we’d probably still have Orohippus and Epihippus running around. Natural selection just ends up promoting the particular individuals that work best in the current environment, because they’re the ones that live long enough to have kids and pass on their genes.

·  Evolution doesn’t happen in a straight line. One particular species can branch out into several different species in time. (We’ll talk later about exactly how this can happen.)

·  It’s incorrect to say that horses evolved from zebras, or that zebras evolved from horses. They both evolved from an ancestral Equus species (or from Hyracotherium, if you want to go back that far.) Likewise, it’s incorrect to say that humans evolved from chimps. Both humans and chimps evolved from our equivalent of Hyracotherium.

·  We’ve never found any horse-ancestor fossils in Australia or Antarctica, which makes sense since there hasn’t been a piece of land connecting the other continents to these two in the past 50 million years. Also, no equine fossils at all appear in South America until about 11 million years ago. Again, this makes sense since the Isthmus of Panama didn’t arise until 11-12 million years ago.

·  An individual zebra can’t evolve – only a population can. I can’t change my genes at will, and neither can you. Organisms become more adapted to their environment ONLY when someone happens to be born with a mutation in their DNA that affects their body or their behavior in a good way. Some species die out because no babies are born with mutations that will help them survive a changing environment better than their parents. Sad, but true. L

·  Remember, all of this happened over millions of years! It takes a long time for a population to collect enough mutations to be considered a different species!!!