Undeniable (10 page)

Our modern domestic dogs are direct, and I mean direct, descendants of either wolves or a common ancestor a step earlier than wolves. Since we have access to machines that use carefully developed reagents to unwind and meticulously count the sequence of nucleic acids—the coding chemicals that are assembled like rungs on a ladder—in an organism's DNA, we can compare directly the DNA of a wolf to that of a New Zealand Blue dog, for example.

Even more remarkable is a set of experiments led by Dmitri Belyaev on a Soviet fox farm during the 1950s. He and his colleagues gained access to a pack of silver foxes, which are prized for their fur and are closely related to wolves. The researchers observed the foxes and offered them human food. The individual foxes that were less inclined to run away as humans approached were rewarded with food. These foxes were selected and allowed to breed, male to female. In just a few generations, the researchers had bred fox pups that were not skittish around the staff. These newly bred fox-dogs or dog-foxes now had an affinity for people. They wagged their tails in joy, whimpered for attention, and licked the experimenters to express affection; some even developed floppy ears, which are scratchable.

The same process must have happened during the domestication of wolves. They were willing to protect our ancestors as though they were one of their own. And this was an apparent result of our providing food and shelter, nice dog-wolf housing. All modern dogs are very nearly wolves, genetically speaking. We humans have decided which wolf-dogs would be allowed to have sex, and so selected the wolf-dog traits that we wanted: friendliness, snuggliness, playfulness, and protectiveness.

Darwin was quite diligent about the business of distinguishing which living things are logically to be grouped with others—specifically which living things were of the same species, able to breed with one another. He observed that what a great many of his contemporaries regarded as different species of roses were, in fact, just slightly different varieties of roses that were easily bred together, or crossed. Darwin's distinction confused a great many people. The roses might look quite different in shape and especially color from one generation to the next. But it is only by seeing if they can breed and produce offspring (more seeds or pips, and more roses) that one can establish whether or not you're dealing with just one or two different species.

Darwin coined the phrase “artificial selection” to describe what human gardeners, farmers, and horse and dog breeders had been doing for centuries: creating better or more useful varieties of animals and plants.

George Washington did it as well. The father of my country spent considerable time, effort, and energy in the breeding of wheat. He used a magnifying glass and tweezers to get the pollen of one wheat stalk to fertilize the eggs or ova of another. Farmers continually control which stud horse gets the gig mating with which mare. If nothing else, they often make male horses into geldings. Ouch. Darwin observed that the processes of agricultural breeding are exactly the same process that takes place in nature. It's just that humans are exerting a great deal of top-down influence. Human farmers and breeders make the decisions as to which genes get selected to pass on to the next generation. Darwin thought of this as gene selection induced artificially (though he did not use the terminology of genetics, which lay far in the future).

On a philosophical note, we still use Darwin's descriptor or adjective: artificial. But notice that from the wheat's point of view, or the horse's point of view, or the point of view of the poodle, artificial selection is the same as natural selection. There you were minding your own business as a wheat plant. Your ova were fertilized by pollen. Wind may have brought them together. A bee may have visited a couple of other wheat flowers and shaken some outsider's pollen onto your eggs. Or a farmer might have carefully shaken pollen from one stalk onto the eggs of another. You, as the offspring plant, cannot tell the difference. From your point of view, having characteristics that appeal to another species is the same whether it's an insect pollinating you or a human.

It is interesting to note that people who believe in creationism, who hold that a deity set up all the ecosystems in just a few days, benefit in every way from our ability to breed better or more useful plants and animals. To get around this obvious contradiction they often resort to elaborate and arbitrary distinctions between what they do and do not call evolution.

Despite the underlying similarity, artificial selection is quite different from natural selection from an observer's point of view. In many cases, artificial selection serves human needs to the exclusion of natural fitness. A reasonable measure of this is whether or not an organism such as a wheat stalk, a soybean plant, a Quarter Horse, or a racing greyhound could or would survive without humans hovering over their affairs, seeing to it that they get fertilized or fed, and controlling their sexual interludes. For three out of those four, the answer is “no.” Humans have made their lives possible. If you consider humans as separate from other living things (if you think a deity enabled and directed us humans to have dominion over Earth), then perhaps we are separate from nature and our genetic manipulations are our divine destiny. If, on the other hand, you see us as part of nature, part of a worldwide ecosystem, then our nominally artificial interference isn't artificial at all. We, and our activities, are part of nature.

Note that just as dogs are still dogs, most of these other domesticated strains still belong to their wild species. A Quarter Horse is still a horse, and spring wheat is still wheat. Artificial selection provides clues about how populations diverge enough to truly become separate species. For example, in the wild when populations of salmon get separated by a change in the flow of tributaries of the same river, the fish might look quite a bit different, one isolated population to the next. But the different isolated populations might still be capable of breeding, were they put back together in a salmon hatchery or separate eddy in the river. Some scientists refer to these as subspecies. If left apart long enough, or if enough generations are hatched separately, eventually we would expect the fish to become unable to breed across populations, and we would declare them different species. We expect that, because every time fish make new fish, every time they reproduce, there will be slight changes in their genes, in their DNA.

Eventually, we would expect too many changes for the subspecies to breed with one another. We should keep in mind, though, that just because we intuitively think of fish breeding as specific to one species and one species alone, it may be that nature allows a spectrum, with some able to interbreed and others not. They may look separate, like different breeds of dogs, while still being a part of the same species. The confusion between outward appearance and inner, genetic nature is also apparent—in a much more pernicious way—in the way people commonly talk about human races. (More on this in chapter 32.) Humans may have more trouble understanding nature than nature does is all I'm saying.

 

11

THE TREE OF LIFE—OR IS IT A BUSH?

People routinely use their “family tree” to describe a diagram of all of their relatives. It's such a common expression that you probably don't even think about the underlying metaphor when you say it. Tracing our ancestry lends itself to depicting one path of lineage. You and any siblings you may have all branch off from your mother and father. If you have children they branch off from you and your partner, and so on. It's a slightly confusing convention, since descendants should intuitively move down (descendants should descend, right?), but a tree grows upward (it ascends). Nevertheless, it's a very useful metaphor, not just for family relationships but for the far broader ones between different kinds of life. So let's climb on that metaphor and keep taking it further, step by step … or rather, branch by branch.

As you climb down from the uppermost tips of your family tree, where you are, to the limb below, and then to the limb below that, you are going to meet up with people who came before you—perhaps ancestors from your grandmother's spouse's side of the family. Farther down, you'll begin to cross paths with strangers; you'll meet people you never met, if you get my drift—ancestors so distant you've never even heard of them. Even farther down the family tree we will come upon the branch where
Homo sapiens
parted ways with other humanlike species. Now the tree begins to include big-picture evolution. If we keep climbing down toward the roots, we come across chimpanzees and orangutans. Below that we would find apes and our ancient common ancestor. Descending farther, we move past the primates and start running into other mammals, including lions, tigers, bears, narwhals, springboks, osprey, and bats.

If we continue our downward climb, we are going to come across branches leading to other organisms that, at first, might not seem that closely related to us (even my old boss…). I'm talking about lizards, fish, and marigolds. By logical extension, we realize that we must all be descendants of a single type of primordial organism at the very base of the tree. I know, I know; it's a little hard to believe at first. But at the microbiological level, we are all much more alike than we are different.

Every organism we can find here on Earth has DNA or its chemical partner, RNA. The acronym RNA is a contraction for ribonucleic acid, and it generally has just one strand compared to the more complex double-stranded structure that is DNA—deoxyribonucleic acid. (“Ribo” derives from an old word for sugar, which was made from a sweet compound called gum arabic. The adjectival form would be arabinose. From there, we end up with “ribo.” Go figure.)

To me, the common code of life raises a powerful question, one that hits me personally. Through a surprising sequence of events in my life, I have become the chief executive officer of the Planetary Society, an organization cofounded by Carl Sagan, who was one of my professors in school. I took the job, because I cannot help but wonder if Earth is special or routine. And so I ask: Is the way life evolved here similar to the way life comes to be on other worlds, or are there drastically different evolutionary paths life can take? Put another way, is DNA, or something much like it, an essential feature of life? The only way to find out is—to find out.

But for certain people who hold a creationist point of view, life's common chemistry paints a completely different picture. They claim it indicates that we are all the product of a designer who made everything according to the same plan, all at once.

That line of reasoning also leads to questions—but they're the exasperating kind. If there was a designer, why did he or she or it create all those fossils of things that aren't living anymore? Why did the designer put all these chemical substitutions of radioactive elements in with nonradioactive elements? Why did a designer program in this continual change that we observe in the fossil record, if he or she assembled the whole system at once? In short, why mess around with all this messiness? If you're a creationist reading this, and you want to remark something like, “Well, that's the way he did it,” I'll tell you right back, that is just not reasonable, nor is it satisfactory. If we were playing on a team right now, I'd say, “Get your head in the game.”

Another thing: If there were a designer, I'd expect some better results. I'd expect no common cold viruses, for example. Or, if viruses are an unavoidable or accidental consequence of a designer designing with DNA molecules, I would hope that we'd be immune to those accidental viruses. If the argument is, “Well, that was all part of the plan,” then I have to ask: How can you take the lack of evidence of a plan as evidence of a plan?
That
makes no sense.

Rather than ascending a tree, I like to think instead of a life moving along a time line, akin to my walk through time across the United States. This “Tree of Life” grows sideways, with time going from the distant past on the left to the present day on the right. Whatever we do to understand the branching pattern of a tree of life, we are working backward. We examine fossils and do our best to assess the age of the rocks in which the fossils are preserved. How else can we do it? We are constrained by the nature of time to start with where we are today and work our way farther and farther into the past, noting branches on a Tree of Life as we come upon them.

The way to peer into life's past is to examine fossils and to determine when a branch was first created by assessing the age of rocks when a certain fossil was formed. The oldest known fossils are of bacteria that apparently lived in ponds or a shallow sea located in what is now western Australia. They are fossil mats of bacteria, called stromatolites, whose metabolism led them to excrete calcium carbonate, chalk, the stuff of seashells and limestone. The ancient ponds dried up for some climatological reason, and these bacterial mats turned to stone. They're 3.5 billion years old.

As we examine rocks that are provably somewhat younger, we find evidence of more complex organisms. When we sample the floor of the ocean down just a few meters, we find a great many deposits of tiny sea creatures. These microfossils are beautiful, complex, cone- and mesh-shaped disks and spirals, which were the homes and bodies of ancient sea creatures. These tiny fossils are often the size of the head of a pin. If you know where to look, you can find hundreds of them in just a cubic centimeter of seafloor sediment. (A cubic centimeter is a milliliter in beer can and baby medicine measurement; it's the same as a cc in old-school medical terminology.)

The farther back in time we carry out an accounting of the number of different living things on Earth, the fewer different kinds we find. The implication is that evolution naturally leads to an increasing number of different kinds of living things. In this way, it is much like a tree. The higher a tree grows, the more branches will grow and bifurcate; each bifurcation leads to another branch and another bifurcation. That's why that Tree of Life metaphor is so powerful.