Lei Chen1,*, Qiang Qiu1,*, Yu Jiang2,*, et al.

Science  21 Jun 2019: Vol. 364, Issue 6446, eaav6202. DOI: 10.1126/science.aav6202

Ruminants are a diverse group of mammals that includes families containing well-known taxa such as deer, cows, and goats. However, their evolutionary relationships have been contentious, as have the origins of their distinctive digestive systems and headgear, including antlers and horns (see the Perspective by Ker and Yang). To understand the relationships among ruminants, L. Chen et al. sequenced 44 species representing 6 families and performed a phylogenetic analysis. From this analysis, they were able to resolve the phylogeny of many genera and document incomplete lineage sorting among major clades. Interestingly, they found evidence for large population reductions among many taxa starting at approximately 100,000 years ago, coinciding with the migration of humans out of Africa. Examining the bony appendages on the head—the so-called headgear—Wang et al. describe specific evolutionary changes in the ruminants and identify selection on cancer-related genes that may function in antler development in deer. Finally, Lin et al. take a close look at the reindeer genome and identify the genetic basis of adaptations that allow reindeer to survive in the harsh conditions of the Arctic.

The ruminants are one of the most successful mammalian lineages, exhibiting extensive morphological and ecological diversity and containing several key livestock species, such as cattle, buffalo, yak, sheep, and goat. Ruminants have evolved several distinct characteristics such as a multichambered stomach, cranial appendages (headgear), specialized dentition, a highly cursorial locomotion, and a wide range of body size variations. Despite their biological prominence and value to human societies, the evolutionary history of ruminants has not been fully resolved, and the molecular mechanisms underlying their characteristics remains largely unknown.

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