Scientists Discover How Cattle and Pigs Evolved Even Toes

When cattle, pigs and other even-toed ungulates evolved, the number of their toes was reduced and transformed into paired hooves. Now, scientists have discovered how this came to be; they've found a gene regulatory switch that was key to evolutionary adaptation of limbs in ungulates.

The fossil record actually shows that the first even-toed ungulates had legs with five toes-just like modern mice and humans. Yet during the course of their evolution, the basic limb skeletal structure changed drastically. In fact, today's even-toed ungulates have just four toes; the second and the fifth toe face backward in pigs, and in castle, the distal skeleton consists of two rudimentary dew claws and two symmetrical and elongated middle digits that form the cloven hoof.

Now, scientists have taken a closer look at how this particular evolutionary adaptation occurred. The scientists compared the activity of genes in mouse and cattle embryos which control the development of fingers and toes during embryonic development.

The development of limbs in both mice and cattle are strikingly similar at first. The differences only become apparent during hand and foot plate development. In mice, the so-called Hox gene transcription factors are distributed asymmetrically in the limb buds, which is crucial to the correct patterning of the distal skeleton. In cattle embryos, their distribution actually becomes symmetrical from the early stages onward.

More specifically, the researchers focused on the Sonic Hedgehog (SHH) signaling pathway, which controls Hox gene expression and the development of five fingers and toes in mice and humans. In the end, they found that the gene expression in limb buds of cattle embryos is altered so that the cells giving rise to the distal skeleton fail to express the Hedgehog receptor.

"The identified genetic alterations affecting this regulatory switch offer unprecedented molecular insights into how the limbs of even-toed ungulates diverged from those of other mammals roughly 55 million years ago," said Rolf Zeller, one of the researchers, in a news release.

The findings show how a simple switch can change the course of evolution. Now, scientists have learned a bit more about how cattle and pigs developed their unique physiology, revealing a bit more about the progress of evolution.

The findings are published in the journal Nature.