A mossy frog, which has bright green skin and lots of spikey skin folds that give it the appearance of being covered in moss, sitting on a rock.

It turns out that frogs might be tens of millions of years younger than previously thought ©Shutterstock/reptiles4all

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Most complete frog family tree shows they evolved later than previously thought

The most comprehensive frog family tree to date is giving researchers a leap forward in their understanding of when amphibians evolved.

By using a combination of fossil and molecular data, the researchers have found that the group may have evolved tens of millions of years later than originally thought. 

Frogs are an incredibly diverse group of animals, found living on every continent except Antarctica.

Since they first appeared, they have evolved to live in a whole range of environments, from the tops of the tallest rainforest trees to burrowing in the soil beneath the roots. This has given rise to an estimated 7,635 species that do a whole variety of things.  

For a while now, scientists have been trying to figure out the exact evolutionary relationships of these frogs, so they can better understand when they started living in these new environments and how they evolved to do it. Initially this was all based on what each species looked like, with those sharing characteristics most likely to be more closely related. But the use of DNA sequencing has meant that researchers could explore these relationships in ever finer detail.

A new study has now taken the most comprehensive collection of frog data from hundreds of genetic markers to create a detailed family tree of frogs. Not only will this provide other researchers with an extraordinary resource, but it is already revealing a few surprising results.

A bright green tree frog with massive black and white eyes is sitting on a little twig.

Frogs have evolved to live in all sorts of environments, including the towering canopies of rainforests ©Shutterstock/Milan Zygmunt

Dr Jeff Streicher is the Natural History Museum’s Senior Curator of Reptiles and Amphibians, and was involved with this new study.

‘Previously the group was thought to have begun to split into the thousands of species we see today around 210 or 220 million years ago,’ explains Jeff. ‘Our new analysis suggests instead that this date was around 180 million years ago.’

‘Finding that frogs are younger means that they diversified into thousands of species more rapidly than was thought before.’

The new frog family tree has been published in the journal Molecular Phylogenetics and Evolution.

What is a phylogeny?

The family tree of animals, known more technically as a phylogeny, is a staple in biological research. First conceived by Charles Darwin, at their very basic they are a diagram that shows the evolutionary relationships of different organisms to each other.

But they can also give researchers deeper insight into the evolutionary history of animals. By adding fossils to the trees, for example, scientists can start to work out the dates that certain groups of animals first evolved. This allows them to pinpoint the relative time in history that animals – or even specific traits – appeared even if there is no direct fossil evidence.

To improve precision about these relationships between species and the groups to which they belong scientists turned to their DNA, which in effect records a history of how related one animal is to another. Think about how DNA tests can be used to figure out parentage.

A very round and grumpy looking rain frog sitting on some brown leaves.

By improving the family tree of  frogs, scientists can then start to look at why and how certain characteristics evolved ©Shutterstock/David Norman Maguire

Due to technological constraints, the first studies could only use a few fragments of DNA from each species. This created trees that contained thousands of frog species, but with poor resolution. As technology advanced, it was ever easier and cheaper to decode longer and longer pieces of frog DNA. This had the potential to revolutionise these frog trees and clear up the initial problems. But it hit a snag.

The newly sequenced strands of genomic DNA were simply too big for computers to be able to cope with when compared across thousands of species.

‘So there were studies using lots of genomic data to resolve parts of the frog tree with unprecedented confidence but they only included a small number of species,’ explains Jeff. ‘This was because we could only analyse so many species worth of data before the computer analysis wouldn’t work anymore.’

Rather frustratingly, when these newer, more high-resolution but fragmented trees were compared to the older ones, they didn’t quite match, and there was no way to be certain which one was most accurate. But over the last few years, computers have finally caught up with the frogs.

‘The big step forward is we were able to get around the computational limitation because we now have access to better servers that let us use more memory than we’ve ever been able to use before,’ says Jeff.

New trees, new dates

The team were able to use the genetic data from 5,242 species of frogs representing an impressive 68% of all known species to create the most comprehensive tree of life for amphibians. This is up from the previous record that involved just over 3,000 species.

While, by and large, the tree looked like how the researchers expected (being something between the older trees and newer ones), the addition of new fossil dates that put the tree in a geologic context has revealed something interesting. 

A diagram of the highly detailed family tree of frogs.

This is the first time that computational power has allowed researchers to look at the frog family tree in such a level of detail ©Portik et al. 

Previous estimates have suggested that frogs first appeared roughly 215 million years ago, towards the end of the Triassic Period when dinosaurs and mammals were also first evolving. This new tree gave new dates, however, that implies frogs may not have appeared until around 180 million years ago, during the start of the Jurassic.  

This shift in time, while perhaps appearing insignificant, could have some knock-on effects on how researchers think different branches of frogs evolved. 

For example, in the temperate rainforests of the northwestern United States is the curious tailed frog, whose closest living relative is found on the opposite side of the Pacific Ocean, living in the forests of New Zealand. At the same time, there are a bunch of frogs in South America which are related to those from Australia.

It was previously thought that the extraordinary distribution of the tailed frog and its New Zealand counterpart was truly ancient, having occurred soon after frogs first evolved during the Triassic. If these news dates are correct, however, this clearly is not possible.

‘Whereas before, the time of divergence between the frogs from North America and New Zealand and those from South America and Australia were thought to be very different, these new dates potentially mean that both divergences may have happened during a single geologic period, the Cretaceous,’ explains Jeff.

While the team can’t say for certain that this is what happened, the resulting tree they have produced will be an incredible resource for anyone wanting to do similar work in the future.