FROM FINS TO LIMBS
Funny bone holds the key to the shift from sea to land
The funny bone, or humerus, is known to most of us because of the unique tingling sensation you feel when it’s bumped. But thanks to its relative abundance in the fossil record, this bone has also helped scientists trace how vertebrates moved from water to land.
A team of researchers at Harvard University and the University of Cambridge analysed 3D models of 40 fossilised specimens that were thought to be alive during the time of the waterto-land transition. You might expect the fossil records to show changes in the humerus that preceded emergence onto land, such as animals with limbs suited to crawling in the mud before they ever left the water, but the team showed this is not the case.
“Just looking at the fossils, you can see the shape changing, but by quantifying in 3D, we can use this to understand how the function was changing across the evolutionary transition,” said lead researcher Dr Stephanie Pierce.
As fins evolved into limbs, the humerus
became more elongated and twisted to support new needs that arose, thanks to the change in living conditions.
“As the form improved for land movement, it had to trade off other
things,” said Pierce. “For instance, the
shape of the bone in a terrestrial tetrapod is actually a much weaker structure than
the fish had originally. But, by creating
this unusual shape, the animal had a longer stride length and could move with more ease – all the things that you want if you’re walking.”
The researchers also found that midtransition species had an L-shaped humerus, an awkward in-between stage. “The earliest tetrapods essentially lost most of the features that were good for swimming, only gaining a few adaptations that were good for walking. They were in this sort of performance valley,” said
Pierce. The researchers now plan to delve
deeper into what they describe as one of the biggest evolutionary transformations that has ever occurred on Earth, to better understand how vertebrates got on land.