Humans share a very old, stocky, and likely slimy ancestor, one that Stephanie E. Pierce, professor of organismic and evolutionary biology, describes as a “short, chunky, croco-salamander.”
More than 330 million years ago, when fish-like creatures started growing legs and taking their first steps on swampy land, they started on a new Darwinian path in the general direction of humanity. But even across that chronological gulf, the fossils of these early animals, called tetrapods, can teach us about our own human bodies, specifically why we grow relatively quickly while other animals, like today’s amphibians, plod along at a slow and steady pace. Salamanders, for example, live as fish-like tadpoles for years before going through metamorphosis to emerge as juveniles where they’ll stay for another 2-3 years.
“Our strategy, not just as humans, but as mammals, is to grow really fast and then reach adult size and slow down,” said Megan Whitney, a former postdoc in Pierce’s lab who is now an assistant professor of vertebrate paleontology and paleohistology at Loyola University. “That strategy was thought to be specialized.” Now, according to new research, humans might not be as special as once thought and the resulting insights might help begin to explain how and why some animals developed a distinct evolutionary advantage on land.
The study was published in Communications Biology by Pierce and Whitney with help from Benjamin Otoo and Kenneth Angielczyk from Chicago’s Field Museum of Natural History. The museum is host to a “giant treasure trove,” as Pierce put it, of fossils from a specific kind of tetrapod known as Whatcheeria. That species was previously thought to be among those that grew slowly and steadily. But, by studying the bones of these ancient creatures, the team discovered that at least some of these croco-salamanders were growing much faster than expected, which may have helped them fulfill the role of an apex predator or even use their stocky legs to make that legendary crawl out of the sea and onto land.
“The water/land transition in tetrapods is one of the icons of evolution,” said Angielczyk. But fish didn’t just sprout legs and crawl on shore; that transition was far from smooth. Some of these early creatures might have evolved sturdier legs that let them both plow through swamps and swim. “There was this burst of experimentation that happened, and some of those experiments worked, and some didn’t,” Angielczyk said.
Judging from Whatcheeria bones, this evolutionary experimentation looks like a mosaic, Pierce said, with some parts of the skeleton seemingly better suited for land and other bits designed for water. That hodgepodge can make it challenging — but also incredibly valuable — to study these animals, which were perched midway through one of the most important evolutionary moments in our lineage. Unlike biologists studying modern animals, Pierce and her team cannot observe Whatcheeria in their natural habitat. To understand how these creatures lived and adapted to a new land-based environment, they search for clues in their bones. And one clue — growth rate — suggests much about how an animal lived and survived.