Using molecular biology to help solve evolutionary puzzles
Combining fossil records with techniques in molecular biology helps reconstruct the evolution of organisms and test hypotheses about where they fit on the tree of life.
Fossils capture a moment in time, providing a glimpse into how organisms once looked and how they have evolved. However, there are few fossil records for some vertebrate groups and that makes their origins difficult to determine. Dr Jakob Vinther, a lecturer in Macroevolution from the School of Earth Sciences, is trying to resolve some of these evolutionary ambiguities using molecular biology.
One method Vinther uses is called a molecular clock. He takes the DNA from modern organisms and based on DNA mutation rates Vinther uses a computer model to calculate when these organisms first diverged: “I have a fossil of a really bizarre looking mollusc that’s extinct but it has plates that resemble those of modern chitons. However, chitons only have eight plates arranged in a single row, whereas this fossil – known as a multiplacophoran – has 17 plates with a very different arrangement."
There are only a handful of fossils that range from 420 million years ago to about 260 million years ago, which then become extinct. The similarities and distinctions of these fossils to modern chitons, have led people to come up with different ideas about how and when they first evolved.
Initially it was hypothesized that multiplacophorans belonged within the group of modern chitons. However, this would indicate that fossils of modern chitons should be found at the same time multiplacophorans first appeared, 420 million years ago (mya). As modern chitons don’t appear in the fossil record until 350 mya, the alternative hypothesis is that multiplacophorans are an early ancestor to modern chitons – a stem group – that evolved chiton-like characteristics separately.
To test these hypotheses, Vinther used the DNA of modern chitons to calculate when they diversified using a molecular clock and found the divergence to be around 350 mya. This finding was consistent with the fossil record and with the hypothesis that multiplacophorans were a stem group, outside the modern chitons, due to their appearance 70 million years before the first modern chiton fossils.
Discovering dinosaurs in colour
Vinther spends a great deal of time looking at exceptional fossils from around the world, trying to understand where they belong on the evolutionary scale. He has an interest in how some of these bizarre looking animals have been preserved – a branch of science known as taphonomy: “I have been looking at some fossils of softbodied animals that have been found in sandstone, which is very rare. These rare fossils lead to questions about the kind of conditions there were to facilitate this preservation.”
During his graduate studies in 2006 Vinther looked at the preservation of squids and their ink sacs, and as part of this research conducted an experiment where he decayed a number of squids. Within days, everything had dissolved into squid slurry with a stench beyond description.
When Vinther realised that the pigment melanin in the ink sacs was preserved in the fossil squids, he began to look at fossil melanin elsewhere. He looked at fossilised feathers and realised that the structures under the microscope thought to be bacteria were in fact pigment-containing organelles known as melanosomes, changing a view held for over thirty years:
“That was a terrible experiment but we found out how to put colours into dinosaurs as a result. It really changed the whole scene in terms of how people saw these fossils, but it also shifted the paradigm that bacteria were of overarching importance in the fossilization process.” - Dr Jakob Vinther
Based on the shape and structure of the melanosomes, Vinther and his colleagues have been able to put colour into the feathers of long extinct animals like Archaeopteryx and the therapod dinosaur Anchiomis.
How feathers evolved: from display to flight
The addition of colour to the reconstruction of extinct animals has allowed insights into their ecology and behaviour. Vinther has examined the structure of feathers and the origins of iridescence to understand feather evolution. Iridescence in feathers indicates a means of display, or as Vinther says: "it implies that the animal was showing off to others. Otherwise, it would be brown like an owl.".
His research suggests display had a big role very early on in dinosaur evolution and perhaps in feather evolution itself. The complex membrane feathers that are associated with flight in modern birds are found very early on in dinosaur evolution, and before we can conceive of a dinosaur flying. The position of membrane feathers on the fore limbs and tails of dinosaurs and their complex colour patterns, has led Vinther to conclude that these types of feathers evolved as a means of display first, then later evolving for flight.
Vinther’s research has helped further our understanding of how and why flight evolved and why feathers are the way they are, but importantly his work is also adding to our understanding of evolutionary processes and relationships between organisms we previously knew little about.