a flock of seagulls

One of the more pleasant aspects of the recent hot weather has been the opportunity, rarely granted to the long-suffering residents of these islands, to sit in the sun outside one’s local pub and enjoy a few beers. During one such sojourn at my own local, I found myself thinking about the Mesozoic era of the Earth’s history. This must sound like a bizarre juxtaposition of ideas, so an explanation of what triggered this train of thought is in order.

My home town, even though it is more than 30 miles from the nearest sea, has a large population of seagulls, many of which could be seen swooping and swerving overhead recently as the pub’s patrons basked in the unaccustomed warmth. Seagulls are highly adaptable creatures, so having any knowledge or experience of the sea is not essential to their continuing survival. Their behaviour is driven by only one imperative: the need for food.

So what’s the prehistoric connection? The Mesozoic era began with the last throes of the Permian mass extinction, 250 million years ago, when more than two-thirds of all land-based vertebrates and 95 percent of all ocean-dwelling species disappeared. During the Triassic period, the first of the three periods that comprise the Mesozoic era, the most significant replacements for these losses were reptiles, in the sea, in the air, and on the land.

In addition to the large reptiles that appeared during this period, there was also a group of smaller reptiles with some mammalian characteristics, known as therapsids. Towards the end of the Triassic, some of these had evolved into the first mammals, which were no bigger than small rodents that were unable to compete with the lumbering leviathans of the Jurassic period. These early mammals were therefore nocturnal, because reptiles are exothermic, meaning that they rely on the environment to maintain their body heat and are thus much less active at night.

Now, if we were to imagine the arrival of extraterrestrial visitors towards the end of the Cretaceous period, 70 million years ago, what would they have made of what they found here? They would not have been able to predict the precise direction in which evolution would have proceeded, but they would have concluded, surely, that the future lay with the big sauropods that dominated every landscape. The rat-like mammals that scurried around would not have registered as having any significance for future developments.

And they might well have been right, had it not been for the arrival of an extremely large meteorite 65 million years ago. When this unexpected projectile crashed into Mexico’s Yucatán Peninsula, the resulting explosion threw up huge quantities of dust. This had two effects: global temperatures plummeted; and the ensuing reduction in light levels severely inhibited plant photosynthesis. Both proved disastrous for the large reptiles: lower temperatures made it almost impossible to maintain body heat, especially if the body in question was extremely large; and reduced levels of photosynthesis meant that much less food was available for the big herbivorous dinosaurs, which in turn meant much less food for big carnivores such as the tyrannosaurs. They died out, leaving the way clear for mammals to take over the world.

We know that this was a worldwide phenomenon, because the thin layer of rock that marks the Cretaceous–Tertiary boundary can be seen on every continent, and this stratum can be linked definitively with a meteorite impact because it contains unusually high levels of iridium, an element that is rare here on Earth but relatively common as a constituent of meteorites.

If we imagine a second group of alien visitors to the planet now, what conclusions might they draw? They would surely note the environmental stresses induced by the dominant lifeform—an expanding population, resource depletion, pollution, etc.—so they might well conclude that, like the dinosaurs, Homo sapiens is doomed to extinction. If that really was their conclusion, they might then speculate on which creatures would fill the niche left behind. And this is where I return to the beer garden of the Agricultural Hotel, and the seagulls wheeling overhead. They have all the necessary attributes: adaptability, small size, audacity and speed of movement. And they will eat almost anything, stealing it if necessary, including food that has been discarded by humans (the main attraction for seagulls around these parts is a large landfill site). They are the ultimate opportunists.

This is all merely speculation, but humans shouldn’t forget that, like the dinosaurs, they haven’t been given any guarantees of their continuing survival, and we could still screw it all up, after which seagulls, or their descendants in the remote future, will inherit the Earth. The meek will have to look elsewhere.