Undeniable (18 page)

Why this debate gets some very, very well established biologists worked up into such a tizzy is a little unclear. It's clear that we need both convergence and contingency. We need to obey the laws of physics and we need to take advantage of opportunities. A look at the species living Down Under shows you what I mean. In Australia, we find counterparts to dogs and cats in the natural ecosystems. There are scavengers that travel in packs, and there are nocturnal hunters that seek food while stalking solo.

Would we naturally have an Australian counterpart to a dog and to a cat if the Australian continent hadn't been connected to the mainland by a land bridge for some time, during an ice age, let's say, when ice and snow held water up and out of the sea? Are a pack hunter and a night stalker needed in every ecosystem? Are they a natural consequence of multicellular animals feeding off plants and things that feed off of plants? Are those ecological niches inherently filled by the nature of life on Earth, or are they just the chance descendants of descendants? If they had been cut off earlier in geologic history, would there be no such counterparts?

To me, it's obvious that both effects are in effect. We cannot say whether four legs or eight or dozens are inherently the best solution to mobility problems. Instead, we can say that all of the schemes we find extant today are a result of solving the same physics and chemistry problems by playing the hand each of us Earthlings was dealt. But I see no evidence that there was a superpower (a super-dealer?) running the show here. Come to think of it, there is overwhelming evidence to suggest that there isn't, at least when it comes to picking winning and losing body plans and designs.

You can look into this topic further and decide for yourself which factor is more significant: Is it convergent forms like wings and feet, flowers and stems? Or is it contingencies like floods, asteroids, and shifting ice caps? I don't see any reasonable hypothesis that explains things as we see them that doesn't include both convergence and contingency in nature. How else could it be? If things were any other way, things would be different.

 

18

MOSQUITOS IN THE TUBE

The problem with a lot of the evidence of evolution is that it is hard to
feel
. No matter how crisp and clear the ideas may be, even the most beautiful fossils remain static chunks of petrified bone, and DNA is still a mess of invisible molecular code. Wouldn't it be something to watch a new species emerge in front of our eyes, over a single human lifetime—to experience evolution not just as an intellectual analysis, but as an immediate phenomenon that could fly right up and bite you? Well, yeah. That's why I got excited when I discovered a place where that is really happening. The location of this amazing evolutionary showcase? The London Underground, affectionately known as the Tube.

In London, as in so many places we visit, there are mosquitos. In biology, we say class: Insecta, order: Diptera, family: Culicidae (insects with two wings, who are like gnats), genus and species:
Culex pipiens
(they buzz). Unlike many other places in the developed world, London used its subway stations as bomb shelters, back during World War II when the city was being attacked with rockets from Germany. To make them harder to intercept, and to make the whole Nazi campaign that much more demoralizing, the rockets were often launched at night. Londoners avoided collapsing buildings and flying bricks by going underground to the Underground. Thousands of people slept in the tube stations as the bombs fell above. The citizens were attacked from the air—not only with exploding V-1 buzz bombs, but also by buzz-biting mosquitos that followed them into the subway stations. There was nowhere for the citizens down there to run, so the mosquitos had a field day … or field night. Mosquitos generally feed at night, especially at dusk, and feed they did.

The layout and construction of tube stations is such that there are often puddles of water between the tracks and near the cracks. Mosquitos lay their eggs in water … perhaps evidence of their waterborne ancestry. With plenty of blood to suck and a place to raise a family (okay they lay eggs and fly away), the London mosquitos had no real need or pressure to ascend to the surface world and look for birds or some other people to poke with their proboscises. A new population of mosquitos got established, one that lived underground, completely separate from their cousins up above.

An isolated population is where new species are likely to emerge, and that's exactly what happened. In approximately fifty years, a new species of mosquito has appeared in the London Tube:
Culex pipien
has become
Culex molestus
. Translating from the Latin roughly: the “mosquito that buzzes” has, by being an isolated population for long enough, become a new species, the “mosquito that bothers.”

Experiments have been done in which the eggs of one of these two mosquito types was mixed with sperm of the other. Generally, they cannot breed together. It works just once in a while. We are watching, in just one human lifetime, the emergence of a new species. The annoying underground-only mosquitos came to be separate just by not being in the aboveground mix. As they reproduce—that is, as they mix genes and have more annoying mosquito offspring—their genes are not copied perfectly, so eventually, apparently, the undergrounders' genes are different enough to make them un-mateable with their very, very recent ancestors who still live above. Upstairs, the
C. pipiens
feed on birds and people. Downstairs, the
C. molestus
feed on us only. Still, you have to figure the molesters can find the occasional rat down there in the tube tunnels. For now, I guess we let them go at each other.

Although the London Underground mosquitos are becoming a separate species overnight, or almost overnight, as of this writing they are not quite a separate species, not yet. In laboratory tests, occasionally certain undergrounders can successfully mate with certain abovegrounders, even though a majority of each cannot successfully interbreed. Looking at the world with deep time in mind, you can imagine that in just another few decades they will be fully separate. They will be unable to mate with each other under any condition; their genes will be too different. Evolutionary biologists see an important lesson here. Because of mutations, populations diverge genetically as time goes on. We can infer that if we went back far enough, we would find a common ancestor for every living thing on Earth. That is one of Darwin's great insights. That is also the small message of the London mosquitos, writ large.

As you look at these two populations of mosquitos, you can see the divergence happening. You have to figure that the same process has always been happening, at least since life began here on Earth. Separate species arise from ancestors, from parents, from organisms that came just before, and just before those that were just before, and so on, and so on, and so on back into the millennia.

The two types of mosquitos still share a great deal of heritage—not surprising, considering how little time they've had to go their separate ways. But you know what? Humans are still genetically very, very close to their cousins, the chimpanzees and great apes. We are different enough to be considered a separate species; humans do not mate with apes (at least, nobody I know). But anthropologists have found dozens of fossils of individuals who are neither like a chimpanzee nor like us. They're clearly related, with hands and hips and skulls almost like ours but not quite. Were they all completely separate species? Could any of them have bred with modern humans if they lived at the same time? These are not idle questions. They are the focus of some of the most fascinating evolutionary research going on right now.

You've probably heard a good deal about Neanderthal [Nee-AND-er-tall] people or almost-people. Sometime around 500,000 years ago, the ancestors of Neanderthals and modern humans diverged. It's a difficult business making inferences about ancient people from the limited remains available, often just a few fragments of bones and teeth. When I was in school, it was believed that the Neanderthal people were completely different from our kind of people. This is to say, it was believed that they and we were entirely different species and never mingled with each other, much less … um … got intimate. Now the evidence shows that our ancestors and Neanderthals not only were alive at the same time, but apparently they interacted. They may have traded with each other, but that's not all they did. Groundbreaking genetic work by Swedish biologist Svante Pääbo and others shows that the two kinds of protohumans had sex with each other. We were hardly more different than those two breeds of London mosquitos.

Somehow, we all got through it. Humans and Neanderthals were carrying on together as recently as thirty thousand years ago. Apparently, we have enough genes in common to have pulled this off. We were close enough to have had babies. I cannot help but hearken to the time-honored lyrics of the accomplished jazz artist Joanie Sommers, who recorded an uncharacteristic 1962 pop-chart hit with “Johnny Get Angry,” in which she asserts that: “I want a brave man / I want a cave man…” Because we've found extraordinary artifacts in caves in Europe, we often associate ancient people with caves. Just looking at an artist's rendering of the Neanderthal physique, you might easily presume that these males were big and strong, which the protagonist in the song would find appealing. Undoubtedly they were, but Neanderthals also had and needed bigger brains than modern humans to operate their bigger frames. You and I have smaller brains than Neanderthals, but we have smaller bodies as well. Our brain-to-body ratio is just a little bit bigger than theirs was.

The lesson to be learned from our cave-guy and cave-gal ancestors, along with our mosquito nemeses in the London Underground, is that the expression “a separate species” is meaningful only in cases where the organisms diverged from a common ancestor some time ago. The process of evolution produces a genetic spectrum, and we're observing it among mosquitos right now in a big city full of modern humans. It reminds me of the big box of crayons that some of my classmates had in elementary school. There was “red-orange” and “orange-red.” If you visit the Crayola Crayon factory in Easton, Pennsylvania, you can watch the pigments get mixed. You can see that it's just a little bit extra of red that produces red-orange, and a little bit extra of orange that produces today's mango-tango. The difference is slight but readily observable. In the case of species, the difference starts out slight, with just a few genetic mutations. But in the very long run of deep time, the difference becomes extraordinary.

The remarkable thing about the evolutionary process that produces new species is that it's brought on by small, random changes in genes. But then the resulting organism or offspring is confronted with an environment that makes or breaks it. The mutation might be random, but the selection pressures are deadly specific. With remarkable speed, new species are created, and diversity abounds.

Since the emergence of a new mosquito protospecies happened so quickly, I cannot help but wonder if there is a little something extra that the underground mosquitos chanced upon. Are they a little better at finding humans than they are at finding birds to suck blood from? Or was it just easier down there, never having to go looking very far for a host? Every day, the hosts come down the stairs to them. Did they just drift apart genetically? Or is there some other advantage they have? The answer may be a combination of these factors. Considering all that's involved, the rate of change is astonishingly fast. Just think what could happen if every living species could beget a new species every one hundred years!

Given that nature produces new species so readily, it's no wonder that we now have at least 16 million species on Earth, with many biologists speculating the true number is far greater. There might be 100 million species extant right now, with perhaps one thousand times that many having come into existence and disappeared. Even as they went extinct, they gave rise to the organisms alive in the less remote yet distant past, eventually giving rise to us and all the living things we see and dig up.

As you might expect, biologists can take the next logical step. They can infer how long ago what we now consider a species came to be separate, even if they don't have fossils to study. One of the most powerful ways to reconstruct ancestry is to assay the genetic code of two living species, observe and measure their current rate of mutations, and then run the genetic clock backward. The amount of genetic difference reveals how long the two species have been going their separate ways. In some cases, the geneticist can then compare notes with the paleontologists to see what the fossils show. There's some guesswork involved, so you wouldn't expect a perfect match, but most of the time the inferred ages come out very close.

That's another remarkable piece of evidence about how evolution works. Two totally different techniques, two totally different ways of looking at life, give the same chronology of the history of life. And it's abundantly clear with each observation farther into the past that all of us Earthlings have DNA, and we all came to be from a common ancestor—just as Darwin deduced.

In the case of the mosquitos, biologists measure the genetic differences between
Culex pipiens
and
Culex molestus
. They measure their rate of change, and sure enough the genetic clock matches up with a split that occurred right around the time of World War II. In the case of humans, we can measure the DNA of someone alive today, and in certain cases, measure the DNA of someone who died centuries ago. A company called 23andMe will analyze your DNA and reconstruct the broad details of your own personal ancestry. Taking this same basic approach, geneticists have identified widespread human genes derived from Genghis Khan, and tracked cancer genes that originated in the Middle East thousands of years ago and then spread with Jewish populations as they moved around the world. My recent ancestors are from Europe, but my DNA assay shows that I'm substantially Bantu.