Tales of Evolution: How fish conquered the land

07 August 2017
Posted by Lewis Thomson.

The vertebrates are an incredibly diverse group of animals, ranging in size from tiny, eight-millimetre frogs to enormous, 30-metre whales. They inhabit all parts of the earth, from the bottom of the ocean to the mountains and skies. Comprising over 60,000 species, the vertebrates can be split into five main groups: fish, amphibians, reptiles, birds and mammals. This three-part series will explore the evolution of the vertebrates, by looking at some of the key evolutionary transitions that took place. Here in part one, we’ll be thinking about how vertebrates moved from the sea onto land, in the fish to amphibian transition.

Vertebrates first evolved around 525 million years ago (mya), and these early vertebrates were all fish, living in the seas and rivers. It wasn’t until 360 mya that vertebrates began to colonise the land. These land animals, called ‘tetrapods’ (meaning four-footed), would give rise to all amphibians, reptiles, birds and mammals - including us! The first tetrapods were similar to modern amphibians, having a flatter skull (from top to bottom) than their fish ancestors, limbs rather than fins; and a neck separating the head from the body. The transition from life in the sea to life on land was undoubtedly a huge step in life’s history. But how did it happen? First, we have to look at what the fossil record has to tell us…

Fossil Finds

In 2004, a team of researchers discovered three fossilised skeletons of an animal which bore some similarities to fish, and some similarities to early tetrapods. The animal, named Tiktaalik roseae, lived 375 mya - just 15 million years before the early tetrapods. It was between one and two metres long, had a flat skull with eyes and nostrils on top, a neck and limbs with a bone structure somewhere in between that of lobe-finned fish fins and early tetrapod limbs. Tiktaalik roseae is one of several species that shows tetrapods evolved from the lobe-finned fish - the group of fish that includes coelacanths and lungfish. Unlike most other fish, in which the fins connect to the trunk via long, thin rays of bone, the fins of lobe-finned fish connect to the trunk via a single, thick bone - similar to the limbs of tetrapods. It is likely that Tiktaalik roseae spent most of its time in shallow waters, using its sturdy fin-limbs to occasionally pull itself up onto land - possibly to escape predators, find prey on land, or to drag itself across land from one swamp to another. Fast forward ten million years to 365 mya, and you begin to see creatures like Ichthyostega and Acanthostega with limbs that are more similar to ours - with the upper limbs containing a humerus, radius and ulna, and proper digits.

Fin to limb evolution: The pectoral fin of a lobe-finned fish Panderichthys (380 mya) is shown on the left. The upper limb of early tetrapods Acanthostega and Tulerpeton (365 mya) are shown on the right. The fin-limb of Tiktaalik (375 mya) is an intermediate between the two types. Image redrawn from (5)

The fossil record gives us a good idea of how the skeletons became better adapted to life on land - with a slow transition from sturdy fins to proper limbs. However, there is a slightly bigger problem of adapting to life on land, and one that the fossil record struggles to answer. This is the problem of how to get your oxygen. Most fish do this by drawing water in through their mouth, and then pumping this water out again through the gills. As the water passes through the gills, oxygen from the water is absorbed into the bloodstream, and carbon dioxide exits from the bloodstream into the water. We tetrapods, on the other hand, draw air in through the mouth or nostrils, into the lungs. A similar exchange of oxygen and carbon dioxide takes place between the lungs and the bloodstream, and then the air is breathed back out again. So how did lungs and air-breathing evolve? We know that Tiktaalik roseae and Ichthyostega had sturdy ribcages - and therefore likely possessed air-breathing lungs. However, apart from this, there is little the fossil record can tell us, and so we must turn instead to developmental biology.

1280px-Acanthostega_model.jpg

Acanthostega reconstruction at the State Museum of Natural History in Stuttgart, Germany

Clues from conception

In humans, the lungs begin to develop about five weeks after conception. They develop as a single bud which grows out from the gut tube. This bud then grows and branches into two to form your right and left lungs. Although the lungs and the gut are used for very different things it makes perfect sense that they develop from the same tube, given that they are still connected in your body where the oesophagus (food pipe) and trachea (windpipe) meet in your throat. But what does lung development tell us about lung evolution? Interestingly, many fish possess a structure that develops in a very similar way. This structure is called the swim bladder, and it also develops as a bud which grows out from the gut tube - although in most fish this bud then splits off from the gut tube. Blood vessels line the outside of the swim bladder and deposit gases into the swim bladder, and the gas-filled swim bladder helps keep the fish buoyant. However, in some fish, the swim bladder doesn’t split off from the gut tube - it stays connected. This means, instead of depositing gases into the swim bladder from the blood vessels, the fish can swim to the surface and gulp air directly into the swim bladder, and also expel air out of the swim bladder if it needs to reduce its buoyancy. In other words, these fish have an air-breathing gas-exchange organ which develops in the same way to our lungs! Lungfish, which are lobe-finned fish, go a step further and use this organ to take up oxygen into their blood for respiration - in exactly the same way as tetrapods do!

It seems likely then that lungs evolved first in the lobe-finned fish, and some of these lobe-finned fish evolved sturdier fins which allowed them to pull themselves up onto land. Combining the ability of air-breathing with locomotion on land, these fish were able to exploit a whole new environment - free from predators, and full of insect prey. These fish became tetrapods, and over the next 360 million years they diversified into all of the amphibians; reptiles; birds and mammals we have today.
 

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