Image created by Dr Sebastien Puechmaille, UCD School of Biology& Environmental Science
By applying 21st-century genetic technology to our understanding of evolution, an international team involving UCD has highlighted important steps in how present-day mammals - including ourselves - arose. And their research, which uses genetic differences as a telescope back into time, sheds light on how the demise of the dinosaurs 65 million years ago was linked with the rising success of the mammals.
The study, published recently in Science, builds on over a decade of work and draws together data and expertise from many different fields around the world, explains Dr Emma Teeling, a lecturer at UCD School of Biology & Environmental Science.
“Traditionally people would use bones, teeth and limbs to work out the evolutionary history of organisms, make a phylogenetic tree, a diagram, a pathway of understanding how things evolved from each other,” she explains. “But we were able to use DNA to do this - we would look at the parts of the gene that were the same, look at the parts that were different. Using this type of information you can draw trees - it’s particularly powerful if you can couple them with the fossil record to try and estimate the mutation rates at different nodes and work out when these things happened.”
Dr Teeling now heads up the Molecular Phylogenetics and Mammalian Evolution lab in UCD, but she has been researching this approach since her PhD at Queen’s University Belfast, and she works with many other leaders in the field, including William Murphy at Texas A&M University and Mark Springer at the University of California, Riverside, who led the recent study.
In 2001, Teeling and collaborators published their initial findings about the ‘phylogenetics’ of mammalian evolution in the prestigious journals Science and Nature. “That really revolutionised our understanding of how mammals evolved, and man’s place in the mammals,” recalls Dr Teeling. So the researchers intensified the effort in a more encompassing project, aiming to build up a broader and deeper picture of mammalian evolution.
Dr Teeling, who directs the Centre for Irish Bat Research, provided the data and analysis on bats, a complex grouping that she admits threw up particular challenges, even when the information from genes and fossils was plentiful. But by carefully choosing genes to analyse, she and the other researchers were able to look at the rate of mutations within particular groups of mammalian species and work out when and how they evolved.
And in recent years, the technology has become much more affordable to take this approach, notes Dr Teeling. “We were lucky that due to the advances in molecular technology, and the drop in price of sequencing DNA that we were able to generate loads and loads of data. Plus now we have supercomputers - so analyses that used to take years are now possible within a week or two.”
So what did they see through this more powerful genetic telescope? Two major pulses of diversification in mammals, each at times when the Earth’s environment and ecology was changing. “We looked at peaks of diversification and radiation over time, and we saw one pulse around 80-82 million years ago, right at the end of the Cretaceous terrestrial revolution, when lots of orders were splitting from each other,” says Dr Teeling.
“This was the time when flowering plants were diversifying - we would argue that the plants, the real drivers of change, diversified and this allowed opportunities for small mammals to diversify. It’s always wonderful when you see an event in your phylogenetic timetree and it matches to something huge that was going on.”
There was another surge in diversification among mammals when the dinosaurs went extinct, just over 65 million years ago, notes Dr Teeling. “Most of the mammalian orders had already started to diverge from each other,” she says. “But they didn’t diversify into their modern orders until the dinosaurs went extinct.” The study’s findings bear a cautionary tale about the impact of climate change and the enormous impact it can have on the planet’s life, she adds.
Having a better understanding of the phylogenetic tree of life can also help to identify important genes in health and disease, and a strand of Dr Teeling’s work, which is funded by Science Foundation Ireland, looks at genes related to inherited deafness and blindness. “You have to work out which part of a human gene doesn’t work in these conditions, which is very difficult to do,” she says. But she explains that comparing human genes with the corresponding genes in other species can help narrow the search for genetic locations where changes could have a huge impact on disease.
The research team behind the recent phylogenetic mammalian study in Science is continuing to build up the data, which should give us further insights into the matrix of evolution and help inform biology and medicine today, according to Dr Teeling.
“If we are really trying to understand adaptation and can understand the underlying mechanisms that make you what you are, makes you sick, makes you respond to environment, makes you attract your mate then we can really understand how life works,” she says. “But you have to understand how it evolved first, and this is a step in the right direction.”
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