Undeniable (19 page)

By learning how our ancestors' DNA was configured we can get also some sense of where human evolution is headed now. As we'll see, there are other ways to figure out how humans are currently evolving. Our descendants may puzzle over how we missed some obvious aspects of evolution, ones that would have helped us make better decisions as a society.

Perhaps one of the most important insights is that humans are extremely uniform genetically. We are just emerging from our own genetic bottleneck. As a result, we are as much like Neanderthals as those two breeds of mosquitos are like each other. Everyone living today is much, much more closely related than that, even. Perhaps that sense of shared heritage can help motivate us to work together more often and accomplish great things.

 

19

CONVERGENCE, ANALOGY, AND HOMOLOGY

People love to find patterns. It seems to be hardwired into our brains. We do it all the time. We say, “this flower looks like that kind of flower,” or “that cloud looks just like a dragon.” Perhaps the ultimate in groupings is the time-honored phrase, “tastes just like chicken.” Pattern recognition undoubtedly aided our survival by helping us recognize good foods, dangerous predators, family members, and so on. When it comes to understanding evolution, though, this tendency has some strange consequences. It tricks us into seeing relationships where there are none, and missing ones that are not written obviously on the surface. On the other hand, it intuitively exposes the physical laws that shape so much of natural selection. I suspect that you know things about evolution that you don't even know you know.

The human penchant for pattern seeking is what led early naturalists to group organisms by how they appear. It was a start in making sense of life's bewildering diversity. You don't have to be an especially disciplined naturalist to infer that although they both fly, bees are fundamentally different from birds. You could also probably easily come up with the idea that birds and bats are somehow more closely related to each other than they are to bees. With just one more step—or flap—you can tell that penguins are more closely related to crows than they are to crawfish, even though two out of those three swim. This impulse toward morphological classification is what inspired the Swedish zoologist Carl Linnaeus to create the naming convention that biologists still use to categorize every known species.

But it's also not hard to imagine a different system of classifying living things based on what they do rather than what they look like. Bees, birds, and bats can fly. Their wings are rigged up in very different fashions, but each of these animals, and related groups of animals, has analogous structures that address a similar problem. They have to get around to eat and mate and reproduce. They all converged on the same or a similar solution to the problem of flying. In biology, we call this convergent evolution, and we say their wings are analogous structures. Look closely, though, and you'll see that they are structurally very different, and they came into being at much different stages in the history of life on Earth.

Making sense of evolution really requires both ways of analyzing and classifying living things. If their physical configurations are different but their functions are similar or even exactly the same, those species are only distantly related. If the configurations of organs and bones are nearly the same, they are closely related even if their shapes are rather different. Genetic analysis is another way to test the degree of kinship between two species, regardless of how similar or different they may appear.

When it comes to flying, every creature faces the same challenges of physics, the same equations of motion and energy. To fly, we have to have air moving downward with enough momentum to support the weight of the flying object, be it a hawk, a yellowjacket wasp, an F-18 Hornet fighter plane, a flying fish, or a vampire bat. Watch a bird, the bigger the better. Their wings move down and back. If you've ever learned to swim the butterfly stroke, it's a similar motion. Instead of scooping up water and shoving it behind them, birds scoop air and force it down and behind them. This gives the animal lift and propulsion. In physics, anything that flows is considered a fluid. Thus water and maple syrup are fluids, but so is air. There's a trick in flying that you're aware of unconsciously or consciously.

When a wing or planar (flat) surface is moved through a fluid, it can develop lift or an upward force just by tipping it. It gets lift when it's given a so-called angle of attack, so long as it's moving. So once a falcon is airborne, it can develop lift by flying into the wind. It can climb using the energy of the wind and the angle of attack of its wings. An important consideration for any aircraft, be it nature-made or human-made, is that it be lightweight. Birds keep their weight down by having hollow bones and most of their wing area comprised of feathers. Made of a material very similar to your fingernails, feathers are remarkably strong for their weight, and they are grown so that they interlock and form a very nearly continuous surface. With the muscles below the attach points of their feathers, birds can pull the feathers down tight so that little air can pass between each feather. Or, birds can splay their feathers out like propeller blades or a fan. Each wingtip feather can provide some forward motion, while the inboard feathers provide lift. Birds can control the configuration of each feather and achieve remarkable efficiency.

Bee wings don't work this way, at least not exactly this way. Instead of having several feathers along a wing, each of which can be twisted to present a different angle to the air, bees have four wings. If you look closely, you can see the two pair. Furthermore, as soon as a bee gets in flight, the wings on each side of its body hook together with tiny hooks (hamuli, Latin for “hooks”) along the leading edge of each rear wing to form essentially one wing on each side of her body. (Most of the bees we see are females.) Bee wings are flexible. With muscles that take up a great deal of the space in a bee's thorax, bees push their wings down and back, not unlike a bird's motion, but then they get wacky.

High-speed photography has revealed that bees turn their wings all the way over as they pull them from the end of their back-and-down stroke forward to start the next wing beat. The motion is hard to believe at first. Try straightening your arm and fingers out to your side. Reach forward with your palm at an angle to the floor or ground, imagine pushing air back and down. When your straight arm is behind you, take it up and twist it, so that the back of your hand is facing the floor and your palm is facing the ceiling or sky. With it still twisted, take it back to your starting position. Now, do that 230 times a second—not per minute, per second. It's astonishing. Bees get a component of lift or upward momentum on both their back or down-stroke and their forward or upstroke. It's crazy in its way. They can pull this off—or actually pull this down, and back, and up, down, and back—because their wings are in sockets that let them twist in this—by human arm standards—exotic fashion. No wonder “Ripley's Believe It or Not” asserted that bees defy the laws of aerodynamics (this book's first paragraph). In the 1960s, bee flight was not yet understood.

Bats are a bit like birds and a bit like bees, wingwise. Like birds, bats cannot turn their wings completely upside down. Like bees after their fore and aft wings are hooked together, a bat's wing is one, albeit flexible, membrane. Bat, bird, and bee wings have this in common, though, the surface they present to the air is curved and flexible. Birds achieve this because each feather is able to move almost independently. Bees' wings are flexible membranes stretched between fluid-filled veins. Bat wings are bat skin stretched over bones. Each wing system scoops air and tosses it aft and down as it flaps. In evolutionary biology we say these wings are analogous. Now, the word
analogous
is regular enough, but here in this discipline it's used in a specific way to describe a specific type of relationship. Bees, birds, and bats use flexing wings to fly. But the wings came into existence by different routes over the course of evolution. They all use the same physics, but they are of very different configurations.

Look inside a bat's wing and you'll see it looks nothing like a fly wing. What it does look like is the bones in your arm. You have a humerus, radius, ulna, five carpals, five metacarpals, and fifteen phalanges. So do bats. And, get this, so do birds. This was one of Darwin's most important observations and insights. These bones are all in the same places in the arm and in the wings. It's just that each bone appears to be stretched or smooshed to fit in with the other bones to form an arm or a wing. In evolutionary biology, we call these structures and configurations homologous. They have the same shape, but they don't quite do the same things. You and I can't fly. Bats cannot play the piano. Birds can sing, but they can't hold a drumstick. (Uh, sorry…) I drew a sketch; it's on the next page.

This business of homology is one of the absolutely most compelling indicators of the process of evolution. Just by looking at our bones, you can tell that we
must
have something in common with bats and birds, and even pterosaurs, the flying reptiles that lived at the same time as the dinosaurs. The configuration of their bones is much like ours. It was a wild time back then: With more oxygen to power metabolisms, pterosaur wings were three times larger than those of the biggest living birds. They were like flying dragons, and yet also a bit like us. We are also a little like bees, but much less so. We have a central nerve running front to back. We have a mouth and an anus. We each have hearts. But otherwise, we're not so much alike. Six legs? With wings? Not my style, Ms. Bee. Fingers and toes? Well, sure Mr. Bat. Bees have analogs with bats. Bats have analogs and homologs with birds—and with us. It's wild. It's evolution.

Both analogy and homology are fascinating and keys to understanding where we all came from. Consider a fish and a dolphin. Fish breathe the oxygen that's dissolved in water. Gasses can dissolve in liquids, like bubbles in beer. They stay in the liquid, “in solution,” until the top is popped. A fish settles for whatever temperature the water around it offers. It runs its metabolism at a speed that the water allows. We say fish are cold-blooded, or exothermic, meaning “temperature from the outside.” A marine mammal like a dolphin is warm-blooded; it has systems to keep itself at almost exactly the same temperature all the time, just like us. It uses calories to maintain that to be sure, but it can metabolize food efficiently, because its digestive chemical reactions are in a warm place. We say it is endothermic (“temperature from the inside”), just like us.

But check them out. Whether they're exo- or endo-, whether they have gills or lungs, they have about the same shape. They have to in order to slip through water with any reasonable efficiency. You might observe that fish tail fins are configured up and down, while whale tail flukes are set up side to side. You might think that they're quite different. Well functionally, not so much. Compare the flounder. It hatches ready to swim with its tail or caudal fin oriented up and down, vertically. But as a flounder matures, it rolls over and can lie flat on the ocean bottom. Its tail fin, along with its entire body, becomes horizontal. A flounder can swim without dragging its tail along the bottom. When we examine the ancestors of whales, on the other hand, we know they left the land and began to swim in the shallows. It's reasonable that horizontal flukes were better for shallow seas—no thwapping the sand as you swim. Later, as their descendants had success hunting in the open ocean, there was no need to twist their tails back. Horizontal flukes propel a whale well enough.

Many times while scuba diving or snorkeling, I've experimented with flipping both flippers together in whalelike fashion. You feel like you're moving a bit faster on the downstroke, but I sense an inefficiency on the upstroke. Whales and porpoises don't seem to have this problem. They aren't flipping with their leg bones the way I try; they flip with their spines. Since my spine is so much shorter than a whale's relative to the length of my body, I can't swim with the same oomph. I don't let it get me down. Even though we have perfectly homologous bones, I'm pretty sure I could beat a killer whale on a morning jog. Although if there were a mile-long pool next to a mile-long running track, the orca would win flukes down.

We can take this thought a step farther … or rather, a few steps backward. When we examine the shapes of extinct marine reptiles like ichthyosaurs (“fish lizard,” roughly contemporaries of the dinosaurs), and even more ancient fish like
Entelognathus
(“full jaw fish,” lived about 400 million years ago), we see the same streamlined hydrodynamic shapes we find in modern sharks, tuna, and orcas. To swim in the sea, you've got to keep things smooth. By the way, swimming in the sea is different from shooting like a rocket. Rockets are narrow-tipped and usually offer a straight tube from the bottom of the nose cone or fairing all the way aft to the tail (empennage). Thick up front and tapering to the tail is a streamlining feature of swimming and flying, when you're moving a lot slower than a rocket. So, fish and airplane wings are thick in the front and taper slowly toward the back.