Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (21 page)

The genetic distinction between “single-celled microbe” and “animal with body” completely broke down thanks to Nicole’s work on choanoflagellates. Most of the genes that are active in choanoflagellates are also active in animals. In fact, many of those genes are part of the machinery that builds bodies. A few examples reveal the power of this comparison. Functions of cell adhesion and cell communication, even parts of the molecules that form the matrix between cells and the molecular cascades that ferry a signal from outside the cell to the inside—all are present in choanoflagellates. Collagens are present in choanoflagellates. The various kinds of molecular rivets that hold cells together are also present in choanoflagellates, although they are doing slightly different jobs.

Choanoflagellates (left) and sponges (right).

Choanoflagellates even give Nicole a road map for comparing our bodybuilding apparatus to that of other microbes. The fundamental molecular structure that makes collagens and proteoglycan aggregates is known from a number of different kind of microbes.
Streptococcus
bacteria—common in our mouths (and, one hopes, rare in other places)—have on their cell surface a molecule that is very similar to collagen. It has the same molecular signature, but does not aggregate to form ropes or sheets as collagens do in animals. Likewise, some of the sugars that make up proteoglycan complexes inside our cartilage are seen in the walls of different kinds of bacteria. Their functions in both viruses and bacteria are not particularly pleasant. They are associated with the ways that these agents invade and infect cells and, in many cases, become more virulent. Many of the molecules that microbes use to cause us misery are primitive versions of the molecules that make our own bodies possible.

This sets up a puzzle. In the fossil record, we see nothing but microbes for the first 3.5 billion years of earth history. Then, suddenly, over a span of perhaps 40 million years, all kinds of bodies appear: plant bodies, fungal bodies, animal bodies; bodies everywhere. Bodies were a real fad. But, if you take Nicole’s work at face value, the potential to build bodies was in place well before bodies ever hit the scene. Why the rush for bodies after such a very long time with no bodies at all?

A PERFECT STORM IN THE ORIGIN OF BODIES

Timing is everything. The best ideas, inventions, and concepts don’t always win. How many musicians, inventors, and artists were so far ahead of their time that they flopped and were forgotten, only to be rediscovered later? We need look no further than poor Heron of Alexandria, who, perhaps in the first century a.d., invented the steam turbine. Unfortunately, it was regarded as a toy. The world wasn’t ready for it.

The history of life works the same way. There is a moment for everything, perhaps even for bodies. To see this, we need to understand why bodies might have come about in the first place.

One theory about this is extremely simple: Perhaps bodies arose when microbes developed new ways to eat each other or avoid being eaten? Having a body with many cells allows creatures to get big. Getting big is often a very good way to avoid being eaten. Bodies may have arisen as just that kind of defense.

When predators develop new ways of eating, prey develop new ways of avoiding that fate. This interplay may have led to the origin of many of our bodybuilding molecules. Many microbes feed by attaching and engulfing other microbes. The molecules that allow microbes to catch their prey and hold on to them are likely candidates for the molecules that form the rivet attachments between cells in our bodies. Some microbes can actually communicate with each other by making compounds that influence the behavior of other microbes. Predator-prey interactions between microbes often involve molecular cues, either to ward off potential predators or to serve as lures enticing prey to come close. Perhaps signals like these were precursors to the kinds of signals that our own cells use to exchange information to keep our bodies intact.

We could speculate on this ad infinitum, but more exciting would be some tangible experimental evidence that shows how predation could bring about bodies. That is essentially what Martin Boraas and his colleagues provided. They took an alga that is normally single-celled and let it live in the lab for over a thousand generations. Then they introduced a predator: a single-celled creature with a flagellum that engulfs other microbes to ingest them. In less than two hundred generations, the alga responded by becoming a clump of hundreds of cells; over time, the number of cells dropped until there were only eight in each clump. Eight turned out to be the optimum because it made clumps large enough to avoid being eaten but small enough so that each cell could pick up light to survive. The most surprising thing happened when the predator was removed: the algae continued to reproduce and form individuals with eight cells. In short, a simple version of a multicellular form had arisen from a no-body.

If an experiment can produce a simple body-like organization from a no-body in several years, imagine what could happen in billions of years. The question then becomes not how could bodies arise, but why didn’t they arise sooner?

Answers to this puzzle might lie in the ancient environment in which bodies arose: the world may not have been ready for bodies.

A body is a very expensive thing to have. There are obvious advantages of becoming a creature with a large body: besides avoiding predators, animals with bodies can eat other, smaller creatures and actively move long distances. Both of these abilities allow the animals to have more control over their environment. But both consume a lot of energy. Bodies require even more energy as they get larger, particularly if they incorporate collagen. Collagen requires a relatively large amount of oxygen for its synthesis and would have greatly increased our ancestors’ need for this important metabolic element.

But the problem was this: levels of oxygen on the ancient earth were very low. For billions of years oxygen levels in the atmosphere did not come close to what we have today. Then, roughly a billion years ago, the amount of oxygen increased dramatically and has stayed relatively high ever since. How do we know this? From the chemistry of rocks. Rocks from about a billion years ago show the telltale signature of having been formed with increasing amounts of oxygen. Could the rise in oxygen in the atmosphere be linked to the origin of bodies?

It may have taken the paleontological equivalent of a perfect storm to bring about bodies. For billions of years, microbes developed new ways of interacting with their environment and with one another. In doing so, they hit on a number of the molecular parts and tools to build bodies, though they used them for other purposes. A cause for the origin of bodies was also in place: by a billion years ago, microbes had learned to eat each other. There was a reason to build bodies, and the tools to do so were already there.

Something was missing. That something was enough oxygen on the earth to support bodies. When the earth’s oxygen increased, bodies appeared everywhere. Life would never be the same.

CHAPTER EIGHT

MAKING SCENTS

I
n the early 1980s, there was tension between molecular biologists and people who worked on whole organisms—ecologists, anatomists, and paleontologists. Anatomists, for example, were seen as quaintly out-of-date, hopelessly entranced by an antiquated kind of science. Molecular biology was revolutionizing our approach to anatomy and developmental biology, so much so that the classical disciplines, such as paleontology, seemed to be dead ends in the history of biology. I was made to feel that, because of my love of fossils, I was going to be replaced by one of those new automated DNA sequencers.

Twenty years later, I’m still digging in the dirt and cracking rocks. I’m also collecting DNA and looking at its role in development. Debates usually begin as either-or scenarios. Over time, all-or-nothing positions give way to a more realistic approach. Fossils and the geological record remain a very powerful source of evidence about the past; nothing else reveals the actual environments and transitional structures that existed during the history of life. As we’ve seen, DNA is an extraordinarily powerful window into life’s history and the formation of bodies and organs. Its role is particularly important where the fossil record is silent. Large parts of bodies—soft tissues, for example—simply do not fossilize readily. In these cases, the DNA record is virtually all we have.

Extracting DNA from bodies is incredibly easy, so easy you can do it in your kitchen. Take a handful of tissue from some plant or animal—peas, or steak, or chicken liver. Add some salt and water and pop everything in a blender to mush up the tissue. Then add some dish soap. Soap breaks up the membranes that surround all the cells in the tissue that were too small for the blender to handle. After that, add some meat tenderizer. The meat tenderizer breaks up some of the proteins that attach to DNA. Now you have a soapy, meat-tenderized soup, with DNA inside. Finally, add some rubbing alcohol to the mix. You’ll have two layers of liquid: soapy mush on the bottom, clear alcohol on top. DNA has a real attraction to alcohol and will move into it. If a goopy white ball appears in the alcohol, you’ve done everything right. That goop is the DNA.

You are now in a position to use that white glop to understand many of the basic connections we have with the rest of life. The trick, on which we spend countless hours and dollars, comes down to comparing DNA’s structure and function in different species. Here is the counterintuitive bit. By extracting DNA from
any
tissue, say the liver, of different species, you can actually decipher the history of virtually any part of our body, including our sense of smell. Locked inside that DNA, whether it comes from liver, blood, or muscle, is much of the apparatus we use to detect odors in our environment. Recall that all our cells contain the same DNA; what differs is which bits of DNA are active. The genes involved in the sense of smell are present in all of our cells, although they are active only in the nasal area.

As we all know, odors elicit impulses in our brains that can have a profound impact on the way we perceive our world. A whiff might lead us to recall the schoolrooms of our childhood or the musty coziness of our grandparents’ attic, each occasion bringing long-buried feelings to the surface. More essentially, smells can help us to survive. The smell of tasty food gets us hungry; the smell of sewage makes us feel ill. We are hardwired to avoid rotten eggs. Want to sell your home? It would be far better to have bread baking in the oven than cabbage boiling on the stovetop when prospective buyers come by. We collectively invest vast sums in our sense of smell: in 2005 the perfume industry generated $24 billion of business in the United States alone. All of this attests to how deeply embedded our sense of smell is inside of us. It is also very ancient.