Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (30 page)

The dangling scrotum also serves as a sexual signal in many mammals. Between the physiological advantages of having gonads outside the body wall, and the occasional benefits this provides in securing mates, there are ample advantages for our distant mammalian ancestors in having a scrotum.

Open a shark and you find a huge liver (top). Push the liver aside and you see gonads, which extend relatively close to the heart, as they do in other primitive creatures. Photos courtesy of Dr. Steven Campana, Canadian Shark Research Laboratory.

The problem with this arrangement is that the plumbing that carries sperm to the penis is circuitous. Sperm travel from the testes in the scrotum through the sperm cord. The cord leaves the scrotum, travels up toward the waist, loops over the pelvis, then goes through the pelvis to travel through the penis and out. Along this complex path, the sperm gain seminal fluids from a number of glands that connect to the tube.

The reason for this absurd route lies in our developmental and evolutionary history. Our gonads begin their development in much the same place as a shark’s: up near our livers. As they grow and develop, our gonads descend. In females, the ovaries descend from the midsection to lie near the uterus and fallopian tubes. This ensures that the egg does not have far to travel to be fertilized. In males, the descent goes farther.

The descent of the gonads, particularly in males, creates a weak spot in the body wall. To envision what happens when the testes and spermatic cord descend to form a scrotum, imagine pushing your fist against a rubber sheet. In this example, your fist becomes equivalent to the testes and your arm to the spermatic cord. The problem is that you have created a weak space where your arm sits. Where once the rubber sheet was a simple wall, you’ve now made another space, between your arm and the rubber sheet, where things can slip. This is essentially what happens in many types of inguinal hernias in men. Some of these inguinal hernias are congenital—when a piece of the gut travels with the testes as it descends. Another kind of inguinal hernia is acquired. When we contract our abdominal muscles, our guts push against the body wall. A weakness in the body wall means that guts can escape the body cavity and be squeezed to lie next to the spermatic cord.

Females are far tougher than males, particularly in this part of the body. Because females do not have a giant tube running through it, their abdominal wall is much stronger than a man’s. This is a good thing when you think of the enormous stresses that female body walls go through during pregnancy and childbirth. A tube through the body wall just wouldn’t do. Men’s tendency to develop hernias is a trade-off between our fish ancestry and our mammal present.

The descent of the testes. During growth, the testes descend from the gonads’ primitive position high up in the body. They end up lying in the scrotum, which is an outpocket of the body wall. All of this leaves the body wall of human males weak in the groin area.

MICROBIAL PAST: MITOCHONDRIAL DISEASES

Mitochondria exist inside every cell of our bodies, doing a remarkable number of things. Their most obvious job is to turn oxygen and sugars into a kind of energy we can use inside our cells. Other functions include metabolizing toxins in our livers and regulating different parts of the function of our cells. We notice our mitochondria only when things go wrong. Unfortunately, the list of diseases caused by malfunctioning mitochondria is extraordinarily long and complex. If there is a problem in the chemical reactions in which oxygen is consumed, energy production can be impaired. The malfunction may be confined to individual tissues, say the eyes, or may affect every system in the body. Depending on the location and severity of the malfunction, it can lead to anything from weakness to death.

Many of the processes we use to live reflect our mitochondria’s history. The chain reaction of chemical events that turns sugars and oxygen into usable energy and carbon dioxide arose billions of years ago, and versions of it are still seen in diverse microbes. Mitochondria carry this bacterial past inside of them: with an entire genetic structure and cellular microstructure similar to bacteria, it is generally accepted that they originally arose from free-living microbes over a billion years ago. In fact, the entire energy-generating machinery of our mitochondria arose in one of these kinds of ancient bacteria.

The bacterial past can be used to our advantage in studying the diseases of mitochondria—in fact, some of the best experimental models for these diseases
are
bacteria. This is powerful because we can do all kinds of experiments with bacteria that are not possible with human cells. One of the most provocative studies was done by a team of scientists from Italy and Germany. The disease that they studied invariably kills the infants who are born with it. Called cardioencephalomyopathy, it results from a genetic change that interrupts the normal metabolic function of mitochondria. In studying a patient who had the disease, the European team identified a place in the DNA that had a suspicious change. Knowing something about the history of life, they then turned to the microbe known as
Paracoccus denitrificans,
which is often called a free-living mitochondrion because its genes and chemical pathways are so similar to those of mitochondria. Just how similar was revealed by the European team. They produced the same change in the bacteria’s genes that they saw in their human patient. What they found makes total sense, once we know our history. They were able to simulate parts of a human mitochondrial disease in a bacterium, with virtually the same change in metabolism. This is putting a many-billion-year part of our history to work for us.

The example from microbes is not unique. Judging by the Nobel Prizes awarded in medicine and physiology in the past thirteen years, I should have called this book
Your Inner Fly, Your Inner Worm,
or
Your Inner Yeast.
Pioneering research on flies won the 1995 Nobel Prize in medicine for uncovering a set of genes that builds bodies in humans and other animals. Nobels in medicine in 2002 and 2006 went to people who made significant advances in human genetics and health by studying an insignificant-looking little worm (
C. elegans
). Similarly, in 2001, elegant analyses of yeast (including baker’s yeast) and sea urchins won the Nobel in medicine for increasing our understanding of some of the basic biology of all cells. These are not esoteric discoveries made on obscure and unimportant creatures. These discoveries on yeast, flies, worms, and, yes, fish tell us about how our own bodies work, the causes of many of the diseases we suffer, and ways we can develop tools to make our lives longer and healthier.

EPILOGUE

As a parent of two young children, I find myself spending a lot of time lately in zoos, museums, and aquaria. Being a visitor is a strange experience, because I’ve been involved with these places for decades, working in museum collections and even helping to prepare exhibits on occasion. During family trips, I’ve come to realize how much my vocation can make me numb to the beauty and sublime complexity of our world and our bodies. I teach and write about millions of years of history and about bizarre ancient worlds, and usually my interest is detached and analytic. Now I’m experiencing science with my children—in the kinds of places where I discovered my love for it in the first place.

One special moment happened recently with my son at the Museum of Science and Industry in Chicago. We’ve gone there regularly over the past three years because of his love of trains and the fact that there is a huge model railroad smack in the center of the place. I’ve spent countless hours at that one exhibit tracing model locomotives on their little trek from Chicago to Seattle. After a number of weekly visits to this shrine for the train-obsessed, Nathaniel and I walked to corners of the museum we had failed to visit during our train-watching ventures or occasional forays to the full-size tractors and planes. In the back of the museum, in the Henry Crown Space Center, model planets hang from the ceiling and space suits lie in cases together with other memorabilia of the space program of the 1960s and 1970s. I was under the presumption that in the back of the museum I would see the trivia that didn’t make it to the major exhibits up front. One display consisted of a battered space capsule that you could walk around and look inside. It didn’t look significant; it seemed way too small and jerry-rigged to be anything really important. The placard was strangely formal, and I had to read it several times before it dawned on me: here was the original Command Module from
Apollo 8,
the actual vessel that carried James Lovell, Frank Borman, and William Anders on humanity’s first trip to the moon and back. This was the spacecraft whose path I followed during Christmas break in third grade, and here I was thirty-eight years later with my own son, looking at the real thing. Of course it was battered. I could see the scars of its journey and subsequent return to earth. Nathaniel was completely disinterested, so I grabbed him and tried to explain what it was. But I couldn’t speak; my voice became so choked with emotion that I could barely utter a single word. After a few minutes, I regained my composure and told him the story of man’s trip to the moon.

But the story I can’t tell him until he is older is why I became speechless and emotional. The real story is that
Apollo 8
is a symbol for the power of science to explain and make our universe knowable. People can quibble over the extent to which the space program was about science or politics, but the central fact remains as clear today as it was in 1968:
Apollo 8
was a product of the essential optimism that fuels the best science. It exemplifies how the unknown should not be a source of suspicion, fear, or retreat to superstition, but motivation to continue asking questions and seeking answers.