Why mammals have such complex backbones

Compared to other vertebrate animals such as reptiles, mammals have complex and unexpectedly weird backbones.

Their spines are essentially differently shaped bones arranged into sections, which is a key to their ability to move in so many different ways – as diverse as a cheetah running, a human walking, a bat flying and a whale swimming.

A new study in the journal Nature Communications looks at how and why mammals’ backbones became so complex, and how this complexity changed through time.

“We show that increases in complexity were discrete steps like rungs on a ladder instead of a smooth increase like a ramp,” says lead author Katrina Jones, a palaeontologist from the Museum of Comparative Zoology at Harvard University, US.

“Adaptations for high activity levels in mammals seem to trigger these jumps in complexity, and they continue to influence its evolution today.”

Co-author Ken Angielczyk, from the Field Museum of Natural History, US, says the study focuses on how mammals evolved from ancient relatives with simple spines to the complex structures we see now.

“It looks like it’s not just a gradual accumulation of little changes over time – it’s more discrete changes,” he says. “And one of these big changes may be related to changes in how mammals are able to move and breathe that let us be so active.”

The research team wanted to discover how and when mammals and their ancestors first evolved these specialised backbones.

They examined fossil backbones from mammal relatives called synapsids, which lived 300–200 million years ago, and took precise measurements of the bones to determine how the spines were changing over time.

They then fed all the data into a computer program that modelled the different ways that the spines might have evolved. The model revealed that changes in synapsid backbones probably developed in comparatively quick bursts, rather than gradually and over a long time.

Angielczyk points out that evolution is generally such a slow process that even its “quick bursts” can take millions of years.

“It looks fast from our mountain-top view of evolution, but if you were one of these animals, it’s not like your grandchildren would look totally different from you,” he says.

“Basically, big step-wise jumps in evolution mean that the changes that were happening made a big difference in the organisms’ lives, making them better able to survive and pass on their genes.”

Increasingly complex spines were such a good thing for mammal ancestors, the researchers argue, because they were among the changes related to higher activity levels.

Compared to reptiles, modern mammals have very high metabolisms – we have more chemical reactions happening to keep our bodies going – and we’re more active.

In general, mammals can move more efficiently and have more stamina, but those benefits come with a cost: mammals have to breathe more than reptiles do, eat more, and need fur to keep their bodies warm enough to keep their systems going.

“As part of our study, we found that modern mammals with the most complex backbones also usually have the highest activity levels,” says co-author Stephanie Pierce, also from Harvard.

“And some changes in backbone complexity evolved at about the same time that other features associated with a more active lifestyle evolved, like fur or specialised muscles for breathing.”

“We’re interested in the big picture of how backbones evolve, and there are these long-standing ideas about it being related to the evolution of mammals’ respiration, locomotion, and high activity levels,” says Angielczyk.

“We’re trying to test and refine those hypotheses, and to use them to better understand the broader question of how complexity increases through evolution.”

“This study helps us answer an age-old question – how did life become so complex?” says Jones.

“By looking at this example system, we show that discreet changes, when added up over the millennia, can produce what seems at first glance to be a long-term trend. The evolution of complexity is, dare I say it, complex!”