Guest Post: Dr Anieke Brombacher – Evolution and the fossil record: What plankton can tell us?

The oceans hold tight many secrets but also many clues to unlocking the mysteries of the past. One such secret is evolution. While evolution may be a deeply complex process, perhaps many of the puzzle pieces needed to solve the mysteries of evolutionary biology lie with some of the smallest organisms found in the oceans. Can such tiny organisms help us to understand how life on Earth has continually evolved and adapted to survive, to produce the wide variety of flora and fauna we see on Earth today, to enable life to succeed in spite of changing evinronmental conditions? Here to tell us more about how exploring our oceans is helping us solve the mysteries of evolutionary biology is Dr Anieke Brombacher…..

Hello everyone! I am Anieke, and I work as a Research Fellow at the National Oceanography Centre. Originally from The Netherlands, I moved to Southampton four years ago to do a PhD. I successfully defended my thesis last month, and I now work as a Postdoctoral Research Fellow to study evolution using fossil plankton as unlikely, yet crucial study organisms.

Evolution is a fascinating process. Through continuous adaptation life on Earth manages to keep up with an eternally changing environment, producing a massive variety of species able to deal with almost anything. One of my favourite examples of just what is possible starts with a small dog-like mammal called Pakicetus. This 50-million-year-old artiodactyl is thought to have spent a fair amount of its time in shallow water, hunting fish. This strategy proved so successful that its ancestors gradually developed bodies exclusively adapted for swimming, with fins replacing front legs, elongated tails and nostrils at the top of their skull. Today, their closest living relatives continue to roam the world’s oceans, and include the largest animals that ever existed.

Reconstruction of Pakicetus (Source: http://spinops.blogspot.co.uk/)

A big problem with studying the processes responsible for these drastic evolutionary changes comes from the available fossil record. Fossils of mammals, dinosaurs and other vertebrates are so rare that we are lucky to get one or two complete specimens per species, and many species are probably never found at all. This makes it nearly impossible to study the driving processes of evolution. Very little is known about the processes responsible for the selection specific traits, the speed of evolutionary change, and the relative importance of competition and climate.

That’s where the plankton comes in. During my PhD project, I studied a group of single-celled zooplankton called foraminifera. These sand grain-sized creatures live in the upper 300 meters of the ocean and build tiny calcite shells to protect their cell. When they die, these shells sink to the sea floor. There are so many of them that as much as a tea spoon full of ocean sediment contains about a thousand fossil shells. Add that to the fact that foraminifera shells have rained down on the sea floor continuously over the past 100 million years, and we have one of the most complete fossil records on the planet.

Live planktonic foraminifera Orbulina universa (calcite sphere with spines), eating a brine shrimp. The shell is roughly a millimetre across

Now imagine you are studying the evolution of foraminifera. You take one sample from your sediment core every thousand years, over several million years. Then, looking down a microscope at all the fossil foraminifera shells in each sample you can slowly see species starting to change. Some get smaller, some get bigger, some change shape. And gradually, you see new forms emerging.

Evolutionary stages in an Eocene (~50 millions years old) foraminifera species. Note the gradual development of spikes at the shell edges. (Source: Pearson & Coxall 2014)

By measuring every single individual, we have been able to find many previously unknown evolutionary patterns. For example, several independent studies from Cardiff, Stockholm and the US have shown that the speed of evolutionary processes varies: times with hardly any action are followed by intervals of gradual or even rapid change. It was also shown that both climate and competition between species strongly influence evolution, but in different ways: extinctions are mainly caused by climate change, whereas the origination of new species depends mostly on factors such as competition and predation.

Of course, many questions still remain. We don’t know how well these single-celled creatures represent the evolution of more complex plants and animals. However, the processes causing mutations in DNA, which are the basis for natural selection to act on, are likely similar across all living things. Even though we might never know exactly how dinosaurs grew feathers, or why blue whales got so big, we do understand more about the underlying evolutionary processes. And eventually, this will lead us to a better understanding of the processes shaping life on Earth.

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