Her story doesn't involve any borrowed ribs or knowledge-bestowing apples, but she was the female forbear of all horses alive today. Researchers say the Eve of horses lived about 140,000 years ago. Her family tree contains some revealing gossip about when, and where, horses began their relationship with humans.
To understand the story of Horse Eve, you'll have first convince yourself that any group of living organisms has a most recent common ancestor. Think of yourself and a friend. Unless he or she is descended from an unknown, second branch of life that happened to evolve exactly the same way ours did--without ever interbreeding--then at some point in your family tree you must share an ancestor. It might only take a few generations. If you pick someone on the other side of the globe, you'll probably have to go further back to find how you're related. If you pick a gorilla, you'll have to go back about 7 million years. But you'll get there eventually.
We can also find the most recent female common ancestor between two individuals by looking at something called mitochondrial DNA. Nearly all your DNA is packaged inside the nuclei of your cells. But your mitochondria, the engines that power your cells, have their own miniature set of DNA. And since sperm are essentially a nucleus with a tail, they don't carry any mitochondria. This means all your mitochondrial DNA was passed down, intact, from your mother's egg cell. Your mitochondria are clones of hers, as hers are of your maternal grandmother, and so on.
The one female ancestor who passed down her mitochondrial DNA to every human alive today is called Mitochondrial Eve. She lived about 200,000 years ago in Africa. Her male counterpart can be found by tracing Y chromosomes, which are only passed between men.
That's not to say this Eve and Adam were the first humans, or the only humans of their generation, or even lived at the same time (they didn't). But over the millennia, the lineages of their peers have dead-ended.
And now we can get back to the horses. Italian researcher Antonion Torroni and a large group of collaborators sequenced the mitochondrial DNA of 83 horses. These represented a wide range of horse breeds across Asia, Europe, the Middle East, and the Americas.
The researchers found that horse mitochondrial DNA was diverse, falling into 18 major groups. Based on the DNA mutations that had occurred in each of these groups, they could create a family tree of all horses, including the rare Przewalski's horse (a subspecies from central Asia that was never domesticated). At the base of this tree is the mitochondrial Eve of horses.
When did she live? The researchers attached a time scale to their tree by calculating the rate at which DNA mutations accumulate in horses. Working backward, they placed the so-called Ancestral Mare between 130,000 and 160,000 years ago.
It was only about 6,000 years ago that we domesticated horses, breeding them to carry around humans and our stuff. Some other domesticated species--such as cattle, sheep and goats--have low genetic diversity, indicating that a small population was initially used for breeding. But the many genetic groups found in this study all predate the domestication of horses. In other words, the diversity didn't come from breeding; it came before breeding. Many different types of horses from different locations were incorporated into the domestic horse's gene pool.
But remember that, since we're looking at mitochondrial DNA passed down by female horses, we're only seeing half the story. And in fact a previous study of Y chromosomes in horses found the opposite result: There's almost no diversity among the DNA passed down through males.
To get the whole picture (just as to get the whole horse) we need to combine the male and female donations to the story. When horse breeding began in Asia, it seems that only a few male horses were used. "The modern Y chromosomes derive only from the one or few [male] animals which were domesticated first," Torroni explained in an email. "You could imagine that early horse breeders continued to domesticate wild females while they spread geographically with their animals, but not males."
Those early breeders, spreading across Eurasia, must have assumed that only the male contribution was important to maintaining the quality of their stock. They pulled in new breeding mares from the wild, but kept their male lines pure. It would be another several millennia before people were acquainted with the science of sperm and eggs. But those breeders unknowingly worked a lot of genetic diversity into the domestic horse.
And that diversity might have implications for how we breed and take care of horses today. It's possible, for example, that different categories of mitochondrial DNA make horses more or less successful at racing. Such a finding would be big news for the people who make their money breeding racehorses.
Genetic diversity might also help explain the success of feral horse populations around the world. Though these "wild" groups are descended from domestic horses, they're totally fine living, once again, without us.
Achilli, A., Olivieri, A., Soares, P., Lancioni, H., Kashani, B., Perego, U., Nergadze, S., Carossa, V., Santagostino, M., Capomaccio, S., Felicetti, M., Al-Achkar, W., Penedo, M., Verini-Supplizi, A., Houshmand, M., Woodward, S., Semino, O., Silvestrelli, M., Giulotto, E., Pereira, L., Bandelt, H., & Torroni, A. (2012). Mitochondrial genomes from modern horses reveal the major haplogroups that underwent domestication Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1111637109
Image: Wikimedia Commons/David
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