The Accidental Species: Misunderstandings of Human Evolution (9 page)

Natural selection, therefore, does not demand what we from our human perspective think of as “improvement.” To go further, natural selection cannot be seen as evolution’s guiding hand. It has no personality, no memory, no foresight, and no end in view. To be sure, it’s easy to see that natural selection, if left to operate for long enough, will create the branching patterns of the tree of life in much the way that Darwin suspected it did. However, there is nothing in natural selection that allows you to predict any
particular
pattern that it might generate. This marks a crucial distinction between natural selection and earlier ideas of transformation that presupposed a ladderlike scheme with
Homo sapiens
at the top. In natural selection, the pattern we see was not preordained, manifest, or inevitable in any way. Stephen Jay Gould expressed this idea very well in his book
Wonderful Life
—if we could rerun the tape of life, we shouldn’t necessarily expect the same result every time.

I’d like to go much further than Gould did. In a famous scientific paper, Gould and his colleague Niles Eldredge proposed that evolution would not always proceed gradually, according to the “insensible gradations” proposed by Darwin, but might in some circumstances proceed very rapidly, and in other circumstances not move at all.
¹²
This was the “punctuated equilibrium” model of evolution, much debated
ever since. But the arguments about evolution’s speed—and these arguments have been fierce and acrimonious—all rest on the assumption that there is a narrative to be uncovered, a story that might be read from analysis of the fossil record.

However, any patterns that we see in the fossil record are reconstructed by us, after the fact. Because the fossil record is so fragmentary and imperfect (a point that Darwin grasped with his usual percipience), it is easy for us to read into it any narrative we like and assume that this narrative must be the right one. It is only natural for us to compose a story that suits our own prejudices of evolution (driven by natural selection) leading to ever greater refinement. This is, however, a profound misreading of Darwin’s ideas and reflects a failure to understand the uniqueness of natural selection as a mechanism of transformation. With natural selection, no fate is ever inevitable, unless reinforced as such by hindsight.

The blob of protoplasm in Darwin’s proverbial “warm little pond” could have evolved into anything—or nothing. The fact that evolution took the course it did was a result of natural selection acting on it and its descendants, moment by moment, according to the environmental circumstances prevalent at each given instant. Looking back at the course of evolution from our privileged height, we naturally assume that the only course of evolution possible was the one that led to ourselves.

This idea seems to have made insufficient impact among science communicators, members of the public, and even some scientists. In the world at large, many evolutionary transformations and adaptations are assumed to have been imbued with purpose. For example, feathers are seen as adaptations that allow birds to fly, as if flight were somehow the manifest destiny of birds. That this idea is wrong is shown by the evidence, which suggests that feathers evolved many millions of years before birds took to the air, among dinosaurs that patently would not have been able to fly. It is even possible that some dinosaurs, having evolved feathers, lost them again. This kind of backward-reasoning, in which adaptations are seen as having a purpose in some great transcendental game that lasts for millions of years, is also widely seen in schemes of human evolution that suppose, for example, that humans stood on two legs in order to free up the hands for making tools, to nurse babies, and so on.

This style of reasoning, in which evolution is assumed to have a pur
pose or a goal, is naturally accompanied by an assumption of progress, very much in the pre-Darwinian style. The assumption of progression is not only a misrepresentation of evolution, but ignores most of what is actually going on.

When we strip away the assumption that evolution is progressive, we find a different picture, both richer and stranger. Most of what seems to be going on in evolution is not the acquisition of new, improved ways of living, but their wholesale loss. This is quite at variance with the picture of evolution most people have, of a march of greater complexity and improvement—a picture that, as I hope is becoming clear, is sometimes misinformed. The concept of loss is explored in the next chapter.

3
:
Losing It

Evolution by natural selection, then, is not a noble or divine force that carries organisms on tracks of inevitable and inexorable improvement from the past to the future. Once we’ve roasted that old canard and served it up with orange sauce, we can begin to demolish as spurious the case for human exceptionalism.

But there’s a catch—such progressive and inexorable improvement seems to have been precisely what has happened. Over the eons, living things really do seem to have become more complicated. Simple creatures consisting of single cells, such as bacteria, evolved into complicated creatures consisting of trillions of cells, such as human beings. If “improvement” can be equated with “complexity,” then there seems to have been a general trend, throughout the history of life, for complexity to increase.

It is said that it takes just one ugly fact to destroy a beautiful hypothesis—so how fares my contention that natural selection is a consequence of several circumstances acting together only in the here and now, without having any end in view?

There are (at least) three answers to this. The first was very well put by Stephen Jay Gould in his book
Full House
. Yes, complexity has increased—but how could it not? If the earliest life was simple and microscopic, the only way was up. That aside, complexity seems to have been the concern of the rather small subset of creatures that includes ourselves. Even today, most creatures are simple and single-celled, and almost all of these are bacteria. Bacteria swarm on (and in, and around) every surface in uncounted profusion. Anyone who has eaten reheated cooked rice and come down with poisoning by
Bacillus cereus
might be astonished to know that the symptoms of poisoning are apparent only if there are more than 100,000 bacterial cells per gram of food.
¹
This means that you can still swallow hordes of germs—cities, dynas
ties, empires of them—without even noticing, and suffer no ill effects whatsoever. Unbeknownst to our everyday selves, our skins crawl with bacteria, and bacteria in billions infest our guts.
²
Were every living creature counted as an equal, the total sum of nonbacterial living creation would be utterly insignificant. Complex organisms, rather than representing a general trend toward improvement, seem to have been a somewhat esoteric diversion.

Second, it all depends on what you mean by “complexity.” How can such a thing be measured, and can it really be equated with “improvement” in any simple way? The simplicity of bacteria is more apparent than real. Bacterial cells might look simple—they are usually spherical or sausage-shaped, and their innards seem entirely featureless—but they are supremely adaptable. Many have digestions far more robust than the most adventurous gourmand, and can live in conditions that would kill any human being (and virtually anything else) instantly.
³
Bacteria live in the upper atmosphere, and deep underground.
⁴
There are bacteria that live in dumps of toxic effluent and in radioactive waste.
⁵
There are even bacteria so tough that they can survive exposure to the hard vacuum and intense radiation of space.
⁶
My point is that there are other ways of measuring complexity than numbers of cells, or the numbers of different types of cells in any given creature, or elaborateness of construction—in other words, according to the criterion by which we measure all things, that is, ourselves. More than 150 years after
The Origin of Species
was published, we are still wedded to the cosmic urgings of the nature philosophers, and accept it as axiomatic that Man is the microcosm that measures the macrocosm. In terms of chemical complexity, however, bacteria are far more complex than Man.

The third answer is more involved than either of these two, and goes deep into the mechanics of complexity increase.

The evolution of complexity is a hot topic in modern biology. The late John Maynard Smith, one of the finest biological minds of the past century, broke down complexity into a number of discrete steps, each of which had to be overcome before complexity could increase any further.
⁷
These steps included (among many other things) the evolution of very simple bacterial cells into the complex cells with which we are familiar, with discrete nuclei and subcellular compartments. Science needs its visionaries, and few were more visionary than Lynn Margulis,
⁸
who was the first to elaborate the idea that complex cells developed from simple cells working together to such an extent that they merged
to become a single organism.
⁹
This idea, once dismissed as far-out, is now very well established and can be seen in various stages of completion, even today.

In many situations, bacteria of different kinds work together in sheets or mats called “biofilms.”
¹⁰
The first large organisms—reefs and mounds of mineralized bacterial biofilms called stromatolites—are built of colonies of different bacteria working together.
¹¹
Before the evolution of animals that could graze on them, stromatolites were common (they still live in isolated places where the water is too rich in salt or other minerals for other creatures to tolerate), and bacterial biofilms coated the ocean floor.
¹²
Biofilms are still with us, thriving in, among other places, the lungs of people with cystic fibrosis, where they contribute to the deadly pathology of that disease.
¹³

Beyond biofilms, though, there is much evidence that complex cells, such as those that make up our own bodies, were originally formed from associations of several different kinds of bacteria that became so commingled that they could no longer function independently. The mitochondria—small sausage-shaped bodies in all cells—are relatively closely related to a group of bacteria called proteobacteria.
¹⁴
They even retain a vestige of their own DNA. The chloroplasts—the green bodies that give plant cells their green color (which also have their own DNA)—are distant relatives of the free-living, light-harvesting blue-green bacteria that contribute to stromatolites.
¹⁵
The DNA complements of mitochondria and chloroplasts, though, are mere scraps compared with those of their free-living relatives, as most mitochondrial and chloroplast functions have devolved to the nucleus, in which almost all the DNA of cells is archived.
¹⁶
Mitochondria and chloroplasts cannot function as free-living entities. By the same token, the nucleus—possibly the vestige of another kind of bacterium—depends on bodies such as mitochondria for its energy needs. This kind of union, known as “endosymbiosis,” is now known to have happened many times in evolution. There are some algae whose cells bear witness to not just one but two separate, independent symbiotic events,
¹⁷
as if these cells were Russian dolls.

Complexity exists, and complex cells evolved from simpler ones. My thesis that evolution shows no definite trend in the direction of improvement would appear to have run into a sticky patch.
Au contraire
, say I.

I shall explain.

If evolution by natural selection can be said to have any “point” at all, it is that a creature should do all it can to improve the chances of its own offspring living long enough to reproduce. Why, then, would a simple cell, working perfectly well on its own, subsume its life and many of its functions in a larger collective, in whose stake it would have at best a slice of the action, rather than the whole cake?

The reason, I think, is all about energy, economics, and risk. Reproduction entirely on one’s own terms is an expensive and exhausting business, and the expenditure might not always pay off. Economies of scale apply as much to living organisms as to human industry. The net benefits of working together might outweigh those of continuing as an individual, and these benefits, such as gains in overall efficiency, might include surplus resources that allow greater specialization among the members of a collective, which in turn improves energetic efficiency still further.

These rewards might also include the ability, perhaps, to colonize new ecological niches that might be inaccessible to one’s competitors, and to do so speedily and efficiently; and, crucially, therefore, the capacity to perpetuate one’s genetic heritage far more effectively than one might manage if working alone.

To take just one example: the first plants to colonize the land more than 400 million years ago were small, encrusting things. But the competition for soil nutrients and light was so intense that plants soon formed associations with soil fungi called mycorrhizae to help them get the best out of the earth. The mycorrhizae, living around the roots of a plant, would extend that plant’s network into the soil, helping it extract water and vital nutrients. In return, the plants would feed the mycorrhizae the sugars created during photosynthesis. Plants with mycorrhizae would grow better than plants without, colonizing more and different habitats and increasing opportunities for their offspring to grow—and for their attendant mycorrhizae to prosper. In turn, the mycorrhizae would enable the plants to grow in soils in which they might otherwise wither. Today, land plants and mycorrhizae are totally dependent on one another.
¹⁸
Meanwhile, the plants themselves soon evolved specialized cells that created hard tissues capable of supporting stalks and trunks that could grow upward quickly. Within a geological eyeblink, forests of tall trees sprang up, each tree trying to outdo the other for a share of the sunshine. And so below, with the mycorrhizae around the trees’ roots forming a wood-wide web of underground
nutrient transport.
¹⁹
Plants and mycorrhizae have achieved far more by working together than either could have managed alone.