Scientists Compare Limb Proportionality in Jerboas to Humans

Scientists from UCSD, Harvard University and Montana State University published a study that examines how evolution affects limb development in the jerboa, a species of rodent with long hind limbs like those of a kangaroo. Published in this week’s issue of Current Biology, the team’s seven-year study might help us better understand human limb development.

Researchers sought to address how growth rates differ for bones of varying sizes. The team specifically turned to the jerboa due to its drastic changes in limb development throughout its evolutionary history; namely, its transition from a five-toed quadrupedal ancestor to the three-toed bipedal creature of today.

The jerboa allowed the team to understand how limb proportionality is determined among humans. By studying the skeletal anatomy of over 33 different species of jerboa and comparing them to closely-related species of mice, the team found that certain genetic factors may control for individual bone sizes.

UCSD biological sciences professor and lead author Dr. Kimberly Cooper explained how the discovery is an unprecedented find that directly links evolution with limbic structure.

“We found that evolution controls individual bone size,” Cooper told the UCSD News Center.  “That’s huge, because if you think about the genes that control the growth of all of the different bones in our body, that suggests they have to be regulated in very complex ways.”

Dr. Clifford Tabin, Chairman of the department of genetics at Harvard Medical School and co-author of the study, explained how the analysis of limb development among the jerboas led to a better understanding of the evolutionary process.

“We focused on a group of rodents from Asia that have evolved to variously run on the ground, scale trees and/or run or hop on their hind legs,” Tabin told the UCSD Guardian. “This study was important in sorting out the order in which these changes occurred, allowing us to better understand their evolution.”

Andrew Biewener, a professor at Harvard University’s department of organismic and evolutionary biology and co-author of the study, told the Guardian how the team’s finding can aid medical researchers looking for alternative ways to treat osteoporosis and address abnormal limb growth in humans.

“Understanding what genes and growth factors are involved in regulating growth plate cell dynamics could help develop clinical therapies for correcting long bone growth abnormalities,” Biewener said.

The study also provided insight on the types of genes that may control for limb length in humans.

“Humans, and rodents like mice and jerboas, may appear extremely different, but we are built on the same basic plan,” Tabin said. “Understanding the genes controlling (for example) the different sizes of legs in rodents, thus, gives us insight into the types of genes responsible for the same sorts of differences in humans, e.g., some of us having short legs and others having the physique of basketball players.”

Moreover, the study suggested that the transition from a quadrupedal form of movement to a bipedal one in an organism’s evolutionary history — in this case, the jerboa — may be driven by survivability.

“The group of rodents that comprise the family Dipodidae, which includes the jerboa, is a very cool and important example of how evolution has shaped the limb anatomy and transformed the animals’ [sic] locomotory biology from being four-legged [quadrupedal] to two-legged [bipedal],” Biewener said. “Our analyses suggest that the evolution of a bipedal form is linked to reducing the predation risk for these nocturnal animals.”