A (Very) Short History of Life on Earth

The best flyers are very small, microscopic even, and go wherever the wind takes them. Most living organisms are small and have voyaged like this for time immemorial, whether they were spores from the earliest land plants carried on an Ordovician breeze; viruses sneezed from a tyrannosaur’s nostrils; bacteria sloughed from its skin; spiders borne on floating strands of silk; tiny insects – all were and are a large, and largely unconsidered, aerial plankton that floats from just above the ground to the edge of space. A very small organism, or a spore, or pollen grain, needs no special adaptations such as wings to get airborne, when it can be carried many kilometres by the lightest gust.

And that’s the trouble. Aerial plankton is subject to the whims of the wind, and cannot control where it goes. For very small flyers, intent on imposing some direction on their lives, wings are needed. But for something as small as a mote, the molecules of air seem appreciably larger than they do to something as large as, say, a bee, or a fly. To a speck of dust, the air is viscous, like water, or syrup, so flying is more like swimming. The wings of the smallest winged insects are more like bristles than aerofoils and work like oars, rowing them through the air.

For a creature large enough for the pull of gravity to become more significant than the movement of air molecules, the first stage of flight is merely a kind of controlled falling. Such is parachuting. Those parachutists that manage to travel horizontally further than they fall vertically are known as ‘gliders’ – but it is a controlled kind of falling, nonetheless.²²

When the backbone evolved half a billion years ago, it was a structure held horizontally, in tension. In hominins, it moved through ninety degrees, to be held vertically, in compression. No more radical alteration in the engineering requirements of the backbone has happened – since it first evolved, and it can only be regarded as maladaptive – witness that back problems constitute one of the most costly and frequent causes of illness in humans today. Dinosaurs made a huge success of being bipeds, but did so in a different way – they held their backbones horizontally, using their long, stiff tails as counterbalances. But hominins, like apes, have no tails, and achieved bipedality the hard way.

Matters were worse for pregnant females, which had to adjust to an increasingly unstable and ever-changing load – a circumstance that has left its imprint in human evolution. And no wonder, given that for most of human history, adult females – on whom the continuation of the species depends – spent much of their lives either pregnant or nursing.¹⁸ And worse still: hominin legs tend to be longer as a proportion of overall height, than they are in apes. Longer legs make locomotion more energy-efficient, but there is a cost. The foetus is even higher off the ground than it might be, raising the overall centre of mass, and increasing instability.

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[...] Once hominins started on the little-frequented path that led towards bipedality, natural selection ensured that they had to become very good at it, very quickly.

Human walking is one of the great underappreciated wonders of the modern world. Today, scientists are capable of teasing out the structure of subatomic particles, detecting the rumble and squeak of merging black holes millions of light years away, and even peering into the very beginnings of the universe. Yet no robot has been created that can mimic the natural grace and athleticism of an ordinary human being as it walks.

Humans have existed in numbers for so short a time that in, say, 250 million years, the remains of few, if any, will have been preserved. Those future prospectors with equipment of only the most refined sensitivity might – just might – be able to detect faint traces of unusual isotopes to say that, a short way through the Cenozoic Ice Age, something happened, but they might be unable to say precisely what.

Within the next few thousand years, Homo sapiens will have vanished. The cause will be, in part, the repayment of an extinction debt, long overdue. The patch of habitat occupied by humanity is nothing less than the entire Earth, and human beings have been making it progressively less habitable.

The main reason, though, will be a failure of population replacement. The human population is likely to peak during the present century, after which it will decline. By 2100, it will be less than it is today.¹⁵ Although humans will do much to ameliorate the damage done to the Earth by their activities, they will not survive more than another few thousand to tens of thousands of years.