The Accidental Species: Misunderstandings of Human Evolution (5 page)

Let’s try this thought experiment. Imagine that by some divine grace (because you’d get it no other way) you were granted knowledge of the complete history of every individual creature that ever lived—its offspring, its ancestors and descendants—and could draw the true tree of life, that is, what actually happened. Just to make things simpler (and more relevant to our current concerns), you restrict yourself to drawing the true tree of hominin evolution, back to our common ancestor with chimpanzees. I’ve drawn what this might look like in
figure 3
.

Time moves from left to right—the left is long ago, the right is more recent. This, the true tree, is very bushy, as you can see, and most of the branches lead nowhere—that is, to extinction. At first glance it’s impossible to select any one branch as especially important.

But because of your divinely granted complete knowledge, you’d be able to pick out the line that leads, uniquely, to modern humans.
Figure 4
shows the “true tree” with the ancestry of modern humans indicated by a thick line.

Figure 3

Figure 4

It’s important to realize that in reality you could have no absolutely certain knowledge of the true line of human evolution—what really happened so surely that you’d
know
you knew it—unless you had a record of every single hominin that ever existed, and full details of their ancestry back to our last common ancestor with chimpanzees.

Back in the real world, we are left with what few scraps time and chance have left us, and that’s very few indeed. Primatologist Robert D. Martin estimated that we knew perhaps as much as seven out of every hundred primate species that have ever existed, given a few assumptions about the known diversity of fossil primates, and the number of primate species currently living. Martin made his estimate twenty years ago.
²⁸
Given that the amount of ignorance expands with the gain of knowledge (that the more scientists discover, the more they know that they don’t know),
²⁹
that proportion might well have decreased, even though paleontologists have discovered quite a few extinct primate species since then.

In
figure 5
I show what remains of the “true tree” once the majority of its branches and twigs have been pruned, leaving only a few fossils. The deletions are, I confess, not totally random. I have been particularly careful to erase any branching points, as fossils will never come complete with that information.

From this scatter, one could arrange many different sequences of fossils from older to younger, and suggest that this sequence might represent a probable evolutionary sequence between apelike ancestor and modern humanity. However, given the evidence at hand, you could link up more or less any sequence of fossils from this scatter and make other assertions of equal validity—without divine grace, who would know which was more likely to be correct?

Figure 5

In
figure 6
I show three of the very large number of possible trajectories between ancestor (left) and human (right). All use the same scatter of fossils, but the trajectories are different from one another.

The first is closest to the “truth,” but we could never “know” this. The three plots would, I think, have something in common and that’s this: the fossils used as links in the chain would, when arranged in the order the arranger assumes to be correct, show a progressive increase in those features assumed (retrospectively) to be characteristic of humans—more erect posture, larger brain, and so on. At no point would there be a reversal, such that a descendant would be more stooped than an ancestor, or have a smaller brain. Not that such things are not possible—the case of the Hobbit shows that they are—but because the assumption of progress is so ingrained that it would not occur to most people that there might be any other course besides onward and upward.

Perhaps the most important thing to take away from this chapter is that new discoveries challenge our idea of progress—that matters are subject to a continual improvement, the refinement of each stage building on that of the one before in seamless progression. What the conceit of progress tends to ignore is the idea of loss—that many experiments in life were made that subsequently went extinct, and so are left out of the canonical tale of improvement.

More than this, the idea of progress tends to be based on criteria that we decide after the fact according to our prejudices, and which need not be the most important or relevant ones. Because we seem to have larger brains and a more erect gait than earlier essays in the human condition, we always assume that the evolution of humanity is a story that must be told in terms of progressive increases in brain size and stature. This reasoning, however, is circular. We have larger brains than our presumed ancestors, so evolution must be couched in terms
of brain size, so the discovery of creatures living in the past that had smaller brains will naturally confirm our prejudices. For all we know, our picture of human evolution might be better told in terms of, say, changes in the number of kinds of bacteria that live in our small intestines. After all, your body probably contains around ten times as many bacterial cells as human ones.
³⁰

Figure 6

To really get a grip on why evolutionary arguments about human exceptionalism are wrong, you need to have a good understanding of what evolution is—and what it is
not
.

The next chapters offer a handy cut-out-and-keep guide to evolution by natural selection. You might be surprised to learn that evolution by natural selection is far less—and far more—than you thought it was. After that I’ll discuss the concept of loss in more detail, showing how the stories on which we base our fragile suppositions about human exceptionalism are based on very little evidence at all.

2
:
All about Evolution

The word “evolution” is probably one of the most abused words in any argument about science. To some, it is a rallying cry to rationality. To others, it’s a term of abuse, the term “evolutionist” hardly less derogatory than “abortionist.” There can be few other words that get so much mileage while remaining so poorly understood. “When
I
use a word,” said Humpty Dumpty in Lewis Carroll’s
Through the Looking-Glass
, “it means just what I choose it to mean—neither more nor less.” Matters are made worse by the fact that the meaning of the word has changed over time, and remains ambiguous to this day.

When inventing the wheel, it is best to ensure that it is round before deciding what color to paint it. So, before we can get a handle on the word “evolution” in all its protean and subtle variety, one must first understand how it works, on the most basic nuts-and-bolts level. This is why Darwin started
The Origin of Species
by outlining such a mechanism—and not mentioning the word “evolution” at all. Darwin had very good reasons for not using the word in his masterpiece, as I shall explain a bit further on. Until then one might do a lot worse than follow his example.

Like many people these days, we in the Gee household keep chickens in our backyard. The hens are of several different breeds. We started with bantams, small birds whose function is more ornamental than anything else. They don’t lay many eggs, perhaps ninety per bird per year. They are, however, long-lived. At the time of writing, one of our first hens, a Pekin bantam, is four years old and still going strong. Our next two hens, Polish bantams, are almost as old, and in rude and squawking health. We also have several standard-sized hens, which lay more and bigger eggs.

But the prizes for productivity go to those in the flock that started their careers in intensive egg-production facilities. A battery hen can
lay as many as three hundred eggs per year, but at a cost—the hens don’t live long. When a battery hen stops laying regularly, she dies of old age. Battery hens have been bred that way, to invest as much energy as possible into producing eggs, at a cost to their own bodily maintenance. Our first four battery rescues died of old age within two years, and we are now on our second quartet.

All the battery hens have russet feathers and red combs. They look just like the Rhode Island Reds my mother kept when I was a boy. As every backyard farmer knows, Rhode Islands are just about the best hens to keep if you like lots of eggs. These battery birds plainly have Rhode Island in their heritage, but they’ve been turbocharged to ramp up egg production at the cost of virtually everything else. In other words, they have been selected. If farmers depend for their livelihood on selling as many eggs as possible, they will breed future stock from the most productive egg-layers, and make the rest of the hens into cat food. They’d continually breed from the best layers in each generation, until, many generations down the line, they’d have created a new breed of hen that routinely lays many more eggs than any hen in the original flock.

This idea—the “artificial” selection by stockmen intent on breeding hens that lay more eggs, sheep with fleecier fleece, bulls with beefier beef, and so on—is intuitive, makes sense to anybody—and was how Darwin started the
Origin
.

What Darwin did next was a master stroke. Once he’d established artificial selection as an obvious and unarguable phenomenon, Darwin used it as an analogy for what goes on in the natural world. In nature the role of farmers is played by the environment. Creatures won’t be “artificially” selected by farmers for this trait or that, but “naturally” selected by the ever-changing environmental conditions in which they live. If the climate turns cold, those elephants that happen to have more body hair will be more likely to survive than those that are less hirsute—long enough to breed and pass on their hairiness to their offspring, while the baldies devote their energies to keeping warm rather than reproducing. If the climate continues cold, the bald elephants will eventually be replaced by woolly mammoths.

The beautiful thing about natural selection is its simplicity. All it requires to work are four things, three of which are readily apparent with eyes to see. They are heritable variation, the ever-changing environment, superabundance of offspring, and the passage of long periods of time.