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Animals invade the land

The animal kingdom didn’t invade the land until around 50 million years after the plant kingdom. This gave the plants plenty of time to become well established; and to provide an unwitting food supply for the marauding animals.

creepy-crawlies insects large animals (i.e. fish)

Even then it was only creepy-crawlies – arthropods to be precise. These were the ancestors of insects, spiders, crustaceans (crabs etc.) and many other small creatures. Large animals came some 70 million years later still.

Now for a sea creature to develop the capability to survive on land may seem like a very tall order. But in fact it can be done in easy stages. And the right place to do it is something like a tidal river estuary – with plenty of mud. Rotting land plants will have generated plenty of nutrients for the rain to wash down into the water. So estuaries will have been teeming with life, just as they are today. But they will also have been dangerous places, with far fewer hiding places than many other regions.

One potential ‘place of safety’ is very shallow water. Shallow water tends to be severely short of oxygen. We’re talking mudflats here, mudflats in the baking sun. The cool swiftly flowing streams that salmon use for spawning don’t suffer from this problem. A creature that can get at least a little oxygen direct from the air can escape to this place of safety, where its predators can’t follow it. I don’t know about arthropods and insects, but many fish today have primitive breathing equipment to enable them to do this.

There’s also the little matter of the tide. In many places the tidal range will have been many metres, just as it is today. So animals of all sizes will often have found themselves stranded in little shallow oxygen-depleted pools, with the need to survive until the tide came up again.

The ability to spend some time out of the water brings additional benefits. There will have been quite a lot of food in the shallow water and out on the open mud; courtesy of the plants and bacteria.

Once a creature can spend a little time out of the water, the pressure will be on to be able to spend more time, and to be able to move further and further in search for food. This probably wasn’t a problem for the creepy-crawlies. But the larger fish had a lot of evolving to do before they could master the land. Fortunately the fossil record has come up trumps here, as we will see in due course.

Creepy-crawlies invade the land

The first animals of any kind to invade the land were arthropods, things like spiders, centipedes and mites. By chance, the arthropods were already reasonably well suited to a future life on land. They had an almost waterproof shell, presumably for protection. They were strong for their size, and walked about the bottom on good sturdy legs. Unfortunately these early pioneers are known only from fossilised footprints. So we don’t know too much about what they were like.

It is known however that some of them dined off organic debris. This was available underwater as well as in the bogs and tidal margins. And of course there was also a supply ready and waiting among the low-growing plants that had invaded the land 50 million years earlier. So it’s easy to see how they could have been tempted, bit by bit, into ever more hostile territory – adapting as they went.

Their main challenge was to master the art of breathing air. As we’ve seen, this too could be a gradual process. Water in boggy places can be very rich in food but, like the tidal margins, seriously lacking in oxygen. Hence the evolutionary pressure to get oxygen from both air and water. After that came the challenge of protecting oneself against drying out. But again, this could be a gradual process. The better protected a creature was, the further up the beach it could roam in search of food. The basics were already there as we’ve seen.

Once the mites and other small plant-and-debris-eating beasties had established themselves on land, larger carnivorous arthropods could follow them. And they did. Pieces of a scorpion around 9 cm long have been found from these early Devonian times.

Insects invade the land

I’m not at all clear when insects first invaded the land, or even what they were like. I did read that signs of insects on land don’t appear until the late Devonian, some 70 million years after the arthropods made the jump. But more recently I’ve seen reports that they were around soon after the arthropods, around 400 million years ago. And what is more, they seem already to have been flyers (more).

Large animals invade the land

We know far more about how the fish invaded the land. The fossil record has come up with no less than six different species, each being slightly more land-adapted than the last. The entire process was completed within a few million years

This superb diagram comes from New Scientist (9.9.06). The top three fish types provide background information. If you know your fish, then skip this paragraph. As far as I know, they are the only three fish types still around today. The first is the ray-finned fish. The vast majority of fish today are ray-finned. Their fins are relatively flimsy affairs, only really suited to guidance duty. Next comes the lobe-finned fish. Their fins have bones in them, and are strong enough to enable their owners to flop about on the mudflats, as we discussed earlier. (Just to be perverse, the authors chose the coelacanth for the diagram. The coelacanth lives in deep water.) The third fish, despite its name ‘lungfish’, doesn’t actually seem to be part of our story. Its niche today is freshwater lakes and rivers that dry up periodically. The lungfish dig holes in the bottom and hibernate until the water returns. The holes they make are characteristic, and similar holes have been found from the earliest lungfish times (early Devonian).

The next six animals range progressively from fish with slight adaptations to life in the tidal margins, to a true amphibian. The red one in the middle is Tiktaalik. With a name like that, it was clearly found in Inuit territory, north east Canada to be precise. It’s the most recent find, and the most exciting, because it fills a gap that was worryingly large. Tiktaalik was almost exactly midway between fish and land animal.

I’ve not been able to learn much on the breathing front. Lungs are soft tissue and don’t preserve well. However Tiktaalik certainly had gills, and palaeontologists seem to think that it had lungs as well. I think some of the less-adapted animals are thought to have had an air-breathing capability of sorts also.

It has to be said that these six creatures don’t seem to have evolved from one another, because they lived at much the same time. But they could all have evolved separately from lobe-finned fishes. Each then found a niche that suited it for a while – until better adapted animals out-competed them.

Amphibians’ legs stick out at the side, and can only support the animal’s weight by dint of sheer muscle power. So they have to walk by ‘waddling’. This is not a good engineering design, because even standing still is hard work. Small creatures have no problem with it why?. And it’s O.K. even for large ones, if they spend most of their time in the water.

But we can’t say that animals had truly mastered the land until they stopped this waddling – until their legs moved to under the body, as in dinosaurs and mammals. This enables skeleton to support the animal’s weight directly. No muscle power is needed just to keep us up.

This skeleton is from Richard Cowen’s ‘History of Life’. Cowen calls it a ‘crocodile’ from the late Triassic (around 200 million years ago). To be so described, it must have a lot of crocodilian features. But the way its legs are attached is not one of them. It’s just weird. The skeleton may well be able to support the animal’s weight without help from its muscles. But the whole arrangement is still pretty flimsy. However it makes a good deal of sense, as a possible immediate common ancestor of dinosaurs and mammals.

This means that land animals were able to thrive for more than 100 million years, despite the handicap of being waddlers.