Ronald E. Merrill About 2,500 words

Some years ago there appeared a book called After Man: A Zoology of the Future (1981; St. Martin's Press, New York). The author, Dougal Dixon, attempted to extrapolate the future course of evolution. Assuming (as many people expect) that homo sapiens is wiped out by a self-inflicted ecological crisis, what kind of animals might evolve 50 million years from now?

Dixon describes a fascinating and plausible future zoology, including such interesting creatures as the wooly gigantelope, the groath, the predatory horraine, and the night stalker, a gigantic flightless bat. And yet, though many weird-looking animals are described in his beautifully illustrated book, Dixon does not project the development of a new class.

In the hierarchy of biological classification, species are gathered into genera, which in turn are grouped into families, then orders, then classes, then phyla, and finally kingdoms. When we think of "animals", we normally have in mind vertebrates (rather than insects and worms and such). Vertebrates form a subphylum (part of the phylum chordata, which contains also some creatures with rudimentary backbones). There are seven classes of vertebrate: three kinds of fish, amphibians, reptiles, birds, and mammals.

Being vertebrates ourselves, we tend to see evolution in terms of the development of vertebrates, and we organize the earth's past by the appearance of new classes of vertebrates: the Age of Amphibians, the Age of Reptiles, the Age of Mammals. Now the Age of Mammals began 65 million years ago (though the first mammals probably date back as far as 100 million years before that). We might expect, then, that 50 million years from now a new Age would have begun, with a new class of vertebrates appearing and taking the evolutionary lead. Yet After Man does not project any such new class. Dixon assumes that the Age of Mammals continues.

Suppose, however, we assume that a new class will develop. What would it be like? In order to answer this question, we must consider how the existing classes evolved.

Let's begin with amphibians. In the Devonian period, about 400 million years ago, the first vertebrate invasion of the land occurred. Lungfish crawled up out of the water to become the first primitive amphibians. The land was already occupied by life; plants, and invertebrates such as the insects, had colonized land 30 million years earlier. The early amphibians, however, had a clear field in the ecological niches for large and medium-size animals, and they took advantage of the opportunity. The first amphibians "radiated" -- split into many species -- to fill the available niches. Two evolutionary innovations made this possible: lungs and legs. The gills of the fish were modified into organs that could breath air. Meanwhile, the fins grew and developed stronger bones and eventually became legs. (Incidentally, these early amphibians were not related to modern amphibians. They died out completely, and a new group of amphibians evolved from fish to become the frogs and toads and salamanders familiar to us.)

But amphibians were limited in many ways. They could not go far from the water. For one thing, the legs of the amphibian were poorly adapted to long trips on land. Reflecting their recent evolution from fins, they stuck out to the sides instead of being underneath the body where they could support it efficiently. But a more serious handicap was reproduction. Amphibians, like fish, lay shell-less eggs in the water, where they are fertilized by the sperm of the male. So amphibians always had to come back to the water to breed, and their young had to remain water-dwellers, as tadpoles do today, until they were grown enough to move onto the land. Thus amphibians had the advantage of land-dwelling only for part of their lifespans.

There was an opportunity for improvement, and a new class of vertebrate appeared to take advantage of it: the reptiles that we call dinosaurs. In the reptile, the legs moved under the body, making locomotion much more efficient. There was also a reproductive innovation: the shelled egg. The original shells were probably more leathery than hard, but they sufficed to allow reptiles to lay eggs on land, thus cutting themselves free of the water environment. With this innovation, reptiles could move into niches far inland, even deserts.

The dinosaurs were a very successful evolutionary innovation, and the early reptiles quickly (well, quickly in evolutionary terms) radiated into thousands of species. They ranged from about the size of a chicken up to the size of houses. They fairly quickly wiped out the early amphibians. Some, the ichthyosaurs and plesiosaurs, went back to the ocean, competing successfully with fish. Others, such as the pterodactyls, took to the air. The major limitation on the reptiles was temperature; being cold-blooded animals, they became sluggish, or even died, if the temperature became too low. It is likely that the largest dinosaurs evolved as dwellers in relatively cold climates. Since an animal's body generates heat proportional to its mass, and that heat escapes in proportion to its area, the larger an animal is, the less sensitive to cold. Paleontologists used to picture the giant herbivores like Brachiosaurus as dwellers in tropical swamps; the current theory is that they lived in temperate coniferous forests.

Still, there were many ecological niches closed to the cold-blooded dinosaurs. Probably not even the largest of them could survive in arctic and subarctic latitudes, and small dinosaurs would have been sluggish and helpless at night even in the temperate zone. So evolution, about 200 million years ago, began to produce the next class: birds.

Though some reptiles had invaded the air, a number of new innovations were needed to make a really efficient flying vertebrate. A flyer, caught on the ground, loses its advantage; so it must be able to get airborne quickly at any time. This means warm blood is needed; when a predator is coming at you, there's no time to warm up the engines! Some sort of good insulation would be helpful; hence the development of feathers. Feathers also make for improved aerodynamics and control. Warm blood and feathers -- now there were birds.

But birds also introduced another reproductive innovation: the family. Flying is rather more complicated and difficult to learn than swimming or walking. What's more, it requires more complex and sophisticated apparatus. Birds cannot fly right out of the egg; they must be raised by their parents, and fed until they grow big enough to go out on their own. It's true that some modern reptiles watch over their eggs to some extent, and the dinosaurs may have done the same. But probably no reptile ever showed the intensive nurturing of young characteristic of birds.

At about the same time -- 200 million years ago -- another class of vertebrates appeared: the mammals. Like the birds, mammals evolved warm blood and an insulating layer: fur. This allowed them to take over many ecological niches which were closed to the reptiles -- the small animal niches in cold conditions. And small the mammals remained, for a long time.

Though we speak of the Age of Reptiles or Mammals, in fact there was considerable overlap. Both mammals and birds coexisted with the dinosaurs for over a hundred million years. They might still be doing so if the earth had not been struck by an enormous meteorite about 65 million years ago. The earth was covered by a cloud of dust which lasted months, perhaps years. The surface became cold. Plant life was reduced. The large dinosaurs gradually lost their body heat and simply ran down. The ecology was dramatically changed, and mammals were best equipped to deal with the new conditions.

As the class of mammals radiated into new species and took over most of the niches previously occupied by the dinosaurs, they developed two reproductive innovations. The first was the technique of bearing live young. It is probable that the early mammals laid eggs, as a couple of species still do today. The marsupials, and later the true placental mammals, developed the ability to protect their partly-formed young by keeping them within the mother's body. This has its drawbacks; for one thing, it limits the number of young the mother can bear. But in the increasingly fierce competition of evolution, the ability to protect and nurture the young until they had a better chance of survival proved to be a winning strategy. Mammals built on their success by developing the milk glands for which they are named, and by matching and even sometimes surpassing the parental nurturing provided by birds.

As we review this history, we can see that each new class of vertebrate developed to exploit a previously unoccupied type of environmental niche, and did so by developing one or more major biological innovations. We may summarize the history with a little table:

Class Niches Innovations

Amphibians water margins lungs, legs

Reptiles land (warm zones) better legs, shelled eggs

Birds air warm blood, feathers,

nurtured young

Mammals land (all zones) warm blood, fur, live

birth of young, nurtured

young

Now we are in a position to ask: Will there ever be a new class of vertebrates? And if so, what will it be like?

We can expect a new class to develop only if there is a large group of undeveloped ecological niches that it can exploit. What are the possibilities?

How about the arctic wastes? Currently, for instance, the interior of Antarctica has no vertebrate occupation. Here is an empty niche. Unfortunately, there's not only no vertebrate life -- there's no life at all to speak of. Interior Antarctica is so short on biotica, even microorganisms, than it was used as a model for the martian surface in preparation for the Viking missions. We can easily imagine a new type of vertebrate adapted to the extreme cold -- but what would it eat?

What about the deep ocean? Perhaps a new class of fish could develop, specially adapted to the lightless depths. Food is scarce there, but there is a continual though light rain of dead organisms from the surface layer. There are indeed a few species of fish who occupy the deeps, but there don't seem to be large unexploited opportunities down there.

Another possibility is the underground environment. There are burrowing vertebrates such a moles already, but perhaps one could imagine an entire underground empire of new species. Alas, this too looks not very promising. Moving around underground takes a lot of energy -- digging is hard work. It's hard to see much future in it. One might imagine sedentary species -- perhaps enormous creatures which would dig deep to tap the earth's thermal energy -- but they would be more likely to be plants than vertebrates.

What is left? Nothing -- on earth. But what about space? Space is unlimited, and contains limitless ecological niches, at least in principle -- new planets, asteroids, perhaps the open reaches of space itself. Let's assume for the sake of argument that space is the final evolutionary frontier. What would the new class of vertebrate be like that could exploit it?

Well, our new type of creature must, obviously, be able to survive in space -- to live in a vacuum, subjected to heavy radiation and extremes of heat or cold, and able to move around without gravity. These are pretty difficult requirements, but by no means biologically impossible.

More difficult to meet is the simple requirement of being able to get into space. The energy requirements for escaping earth's gravitational field are so enormous that no reasonable metabolism could meet them.

So our hypothetical creatures will have to use some sort of artificial means to enter space, and probably to survive once there. They will have to build spaceships and spacesuits. An obvious biological consequence is that they will have to be highly intelligent. As a result, their young will need an extraordinarily long period of nurturing and education. Say, isn't this beginning to sound familiar?

The fact is, the first species of the New Class already exists; it's called Homo sapiens sapiens. Currently biologists classify it in the family Hominidae, of the order of primates, of the class of mammals -- but that's a mistake.

It's understandable, of course. If biologists had been around to observe the first primitive amphibian species, they would quite certainly have classified it as a fish. It was far more similar to a lungfish than to, say, a frog. It would have seemed ridiculous to give it a whole class to itself.

But a biologist with sufficient foresight could have predicted the conquest of the land and the radiation of that first species into a host of amphibians. In the same way, we can perceive the opportunity for Homo sapiens sapiens to enter and exploit space, eventually to radiate into thousands of species specially adapted for the environments of different planets.

But there is nothing inevitable about it. That initial conquest of the land might have failed. So might our conquest of space. Perhaps we are analogous to the flying reptiles, who almost made it but were defeated, leaving the way clear for the birds to dominate the air.

We are now on the very threshold of space. Will we go ahead and conquer it -- or turn back? If we turn back, then the world visualized by Dougal Dixon may well evolve. Trapped on this planet, Man can expand only by taking over the ecological niches of other species. This is what we have done and are doing. But the extinction of our competitors damages the ecological diversity that we ourselves need to survive. The danger is great.

The evolution of 400 million years has culminated in a question which, very possibly, must be answered by a single generation. Will we be evolutionary flunkouts, doomed to decay and possibly extinction? Or will we go to the head of the Class?