No more mystery meat
A Texas Longhorn (top) and a museum reconstruction of its extinct ancestor, the aurochs. The images are not to scale.
Where's the evolution?
Between 7,000 and 10,000 years ago, people in the Middle East began to domesticate wild aurochsen living nearby through the process of artificial selection. These early humans saw the advantage of maintaining aurochs for their personal use, as opposed to relying on what they could gain from hunting wild animals. As they began to live more closely with aurochsen, they spurred the evolution of their animals by selecting for individuals with particular traits. This may have been done consciously or unconsciously. For example, perhaps naturally docile animals were the only ones that the herdspeople could prevent from escaping, so without anyone intending it, ancestral cattle herds ended up with an unusually high frequency of gene versions that confer a quiet temperament. Other traits that differentiate domestic cattle from aurochs (like small horns) may have been consciously chosen. In either case, humans likely selected which individuals would be part of their herds and which individuals would be mated to maintain the herd all the while, favoring individuals with traits that made them more useful to us. Through many generations of this process, an ancestral population of wild aurochs evolved into the animals that we recognize as cows.
Shortly after the domestication of aurochs began in the Middle East, the same process began on the Indian subcontinent. The results of these independent domestication events were two distinct lineages of cows each drawing on slightly different subsets of the genetic variation present in the aurochs species. Though still closely enough related to mate with one another, the lineages had distinct traits. The Middle Eastern cows (known as the taurine lineage) had a flat back and upright ears. The Indian cows (known as the indicine lineage) had a hump at the shoulders and droopy ears. Because of the conditions in which they were domesticated, the indicine cattle were also hardier and held up better in times of drought and low feed availability.
Soon after domestication, the two newly minted cow breeds began to travel with their herdspeople. Taurine cows were taken to Europe, where they interbred with local aurochs and were further artificially selected into breeds such as the Angus and the Shorthorn. Indicine cows spread throughout India. Both taurine and indicine cows made their way to Africa, where they were mated to each other, forming breeds like the N'Dama.
That accounts for historical cattle herds in Europe, Africa, and Asia, but how did cows come to America? Researchers recently tried to answer this question by investigating the genetics of New World cow breeds. Historical documents reveal that, in 1493, Christopher Columbus picked up cattle from Spain's Canary Islands, sailed across the Atlantic, and introduced those animals to the Caribbean island of Hispaniola. In the next 20 years, this small Caribbean population was supplemented with additional cattle brought by Spanish colonists. Soon the cows were thriving, and the descendents of this ancestral population were introduced to Mexico, Texas, Colombia, and Venezuela. Then, in a move that might seem counterproductive to their thousands of years of domestication and breeding, the cows were allowed to run wild, and for 400 years, experienced the vagaries of natural selection in the wild, as opposed to artificial selection at the hands of people. Because it likely increased the frequency of gene variants that improved the animals' ability to resist disease and survive in harsh western landscapes, this period of natural selection may have been critical to the development of American cattle breeds. During the 1800s, the feral cow populations began to be treated as livestock once again and were bred into Texas Longhorns, Corriente cattle from Mexico, and Romosinuano cattle from Colombia, which are known to be particularly hardy and disease resistant.
When researchers studied the gene variants possessed by those New World breeds, they found that the animals were much like breeds from Southern Europe, from which New World cattle were presumably derived. No surprise there. However, they also found some unexpected gene variants linking cows like the Texas Longhorn to India. The new analysis of DNA from cows all over the world suggested that after taurine and indicine cows interbred in Africa, some of those hybrids were carried over the strait of Gibraltar, probably with the Moors who invaded Spain between the 8th and 13th centuries. The indicine DNA in American cattle seems to have come to them via a long and winding path from cows that sailed with Christopher Columbus, who got it from Southern European cows, who got it from cows who crossed the Mediterranean with the Moors, who got it from their African ancestors, who got it from animals who travelled to Africa across the Sinai Peninsula, who got it from their Indian ancestors, who, presumably got it from the aurochs populations living in that region 10,000 years ago. Perhaps some of the traits that make American breeds so tough and resilient were inherited from their African and Indian ancestors that also experienced selection shaping them to the rigors of a harsh environment.
With the improvement of genetic technology and advances in analytical techniques, biologists have been able to work out the complex evolutionary histories underlying many of the organisms that we encounter on a daily basis and depend on for food and other materials. Researchers have studied cats, dogs, sheep, chickens, rice, cotton, corn, brussel sprouts, and many more. Often the stories of these organisms are deeply intertwined with our own evolutionary history. So, the next time you sit down to a juicy burger (or pull on a cotton sweater, or play fetch with your dog), take a moment to appreciate the long and arduous evolutionary journeys that have shaped the very qualities that make these organisms so appealing to us humans.
Discussion and extension questions
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Texas Longhorn photo by TexasEagle (CC BY-NC 2.0); aurochs photo by DFoidl (CC BY-SA 3.0)
Understanding Evolution © 2017 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California