Fish and Amphibians

Fish dominate the seas, lakes and rivers. The amphibians live in both water and land. The reptiles can survive in the driest places on Earth. The birds rule the skies… and the mammals have insulated their bodies to adapt to every climate.

Humans have used our greater intelligence to overrun the planet.

This astonishing journey was built on a series of key evolutionary steps that helped our ancient ancestors to exploit their environments and overcome huge challenges.

The first of these advances was the development of that, in support – the notochord.

Back in Europe, you can find a creature that represents the next critical step in our story. It lives unobtrusively and often ignored in British rivers. And it sheds light on the challenges those first vertebrates faced.

This is a lamprey. You might think at first sight that it was a kind of fish, but it’s not. It’s something much, much more primitive. It has no fins, and even its tail is nothing more than a flattened blade. But what is most remarkable about it is that it doesn’t really have a true mouth.

Its mouth is just a simple hole with little bristles about it. And it feeds by sucking in water through that mouth and then filtering out little particles of food.

So this little animal takes us right back to the time when the first animals with backbones appeared on Earth. It’s a true living fossil.

The first vertebrates seem to have had the same kind of mouth and they were almost certainly limited to the same kind of simple food. Over time, other forms evolved with different shapes and sizes, many of them rather larger than Myllokunmingia, but all of them had that very simple mouth, an opening at the front of the body as a lamprey has today. If the early vertebrates were going to really take advantage of the variety of food that was available in those early seas, they were going to have to develop a much more complex and powerful form of eating machinery.

Scientists on the East Coast of the United States are seeing evidence of this evolutionary advance, not in fossils but in living creatures.

Maine, New England.

Marine biologists at the University of New England are studying a group of fish with a very ancient ancestry. They build their skeletons with the same strong material that formed the gristly rod as the first vertebrates – cartilage. They are the sharks, skates and rays.

This group appeared among the vertebrates over 420 million years ago. And that means we can use them to examine the development before that split of a remarkable piece of engineering that changed the course of evolutionary history. The jaw.

Dr James Sulikowski “If you look back on the evolutionary tree, you’d find that the jaw is a really important feature to have and it’s one of the features that have made skates and sharks apex predators in the environments in which they live.”

A jaw hinged to the skull brought the new ability to grab food, then rip or grind it into digestible pieces. But where did this amazing piece of equipment come from?

Scientists have found an answer by studying the way living vertebrates develop as embryos. Skates lay their fertilised eggs on the seabed inside leathery cases called mermaid’s purses. Scientists can open these up and observe them as they develop, fed by a generous supply of a egg yolk.

The skate embryo has a simple structure shared by all embryonic vertebrates that served as the basis of the first jaw.

Dr James Sulikowski “What we see are these folds… and what’s really interesting about this, is that this skate is in about… four months of its development. And if we take a close look at another vertebrate, we can see it looks very similar. Here we have the head, as you can follow it down to the body. Also see the folds. Now, this is actually a human being.”

It’s thought that the embryos of the earliest vertebrates looked much like this and that each fold developed into a gill.

In a skate embryo, the folds furthest from the head keep to their original purpose and form the rigid arches of its gills. But the nearest fold has been adapted to form an upper and lower jaw. In a human embryo, the lower folds develop into structures that include the larynx and the throat. But the top fold, once again, constructs the jaw.

The development of the jaw improved the ability to collect food, and those that lacked it, with a few exceptions like the lamprey, died out. But in order to collect food, you have to find it and that led to an improvement in swimming.

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