Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (15 page)

Why stop at frogs and sharks? Why not extend our comparison to other creatures, like insects or worms? But why would we do this when none of these creatures has a skull, much less cranial nerves? None of them even has bones. When we leave fish for worms, we get to a very soft and headless world. Bits of ourselves are there, though, if you look closely.

Those of us who teach comparative anatomy to undergraduates usually begin the course with a slide of
Amphioxus.
Every September, hundreds of
Amphioxus
slides appear on screens in college lecture halls from Maine to California. Why? Remember the simple dichotomy between invertebrates and vertebrates?
Amphioxus
is a worm, an invertebrate, that shares many features with backboned animals such as fish, amphibians, and mammals.
Amphioxus
lacks a backbone, but like all creatures with backbones, it has a nerve cord that runs along its back. In addition, a rod runs the length of its body, parallel to the nerve cord. This rod, known as the notochord, is filled with a jelly-like substance and provides support for the body. As embryos, we have a notochord, too, but unlike
Amphioxus
’s, ours breaks up and ultimately becomes part of the disks that lie between our vertebrae. Rupture a disk and the jelly-like substance of what was once a notochord can wreak havoc when it pinches nerves or interferes with the ability of one vertebra to move along the next. When we injure a disk, a very ancient part of our body plan is rupturing. Thanks a lot,
Amphioxus.

The closest relatives to animals with heads are worms with gill slits. Shown are
Amphioxus
and a reconstruction of a fossil worm (
Haikouella
) over 530 million years old. Both worms have a notochord, a nerve cord, and gill slits. The fossil worm is known from over three hundred individual specimens from southern China.

Amphioxus
is not unique among worms. Some of the best examples are not in the oceans of today but in ancient rocks of China and Canada. Buried in sediments over 500 million years old are small worms that lack heads, complex brains, or cranial nerves. They may not look like much, being small smudges in the rock, but the preservation of these fossils is incredible. When you look under a microscope, you find beautifully preserved impressions that display their soft anatomy in fine detail, occasionally even with impressions of skin. They show something else wonderful, too. They are the earliest creatures with notochords and nerve cords. These worms are telling us something about the origin of parts of our bodies.

But there is something else we share with these little worms: gill arches.
Amphioxus,
for example, has them in abundance, and associated with each arch is a little bar of cartilage. Like the cartilages that form our jaws, our ear bones, and parts of our voice box, these rods support the gill slit. The essence of our head goes back to worms, organisms that do not even have a head. What does
Amphioxus
do with the gill arches? It pumps water through them to filter out little particles of food. From so humble a beginning comes the basic structures of our own head. Just as teeth, genes, and limbs have been modified and their functions repurposed over the ages, so, too, has the basic structure of our head.

CHAPTER SIX

THE BEST-LAID (BODY) PLANS

W
e are a package of about two trillion cells assembled in a very precise way. Our bodies exist in three dimensions, with our cells and organs in their proper places. The head is on top. The spinal cord is toward our back. Our guts are on the belly side. Our arms and legs are to the sides. This basic architecture distinguishes us from primitive creatures organized as clumps or disks of cells.

The same design is also an important part of the bodies of other creatures. Like us, fish, lizards, and cows have bodies that are symmetrical with a front/back, top/bottom, and left/right. Their front ends (corresponding to the top of an upright human) all have heads, with sense organs and brains inside. They have a spinal cord that runs the length of the body along the back. Also like us, they have an anus, which is at the opposite end of their bodies from the mouth. The head is on the forward end, in the direction they typically swim or walk. As you can imagine, “anus-forward” wouldn’t work very well in most settings, particularly aquatic ones. Social situations would be a problem, too.

It is more difficult to find our basic design in really primitive animals—jellyfish, for example. Jellyfish have a different kind of body plan: their cells are organized into disks that have a top and bottom. Lacking a front and back, a head and tail, and a left and right, jellyfish body organization appears very different from our own. Don’t even bother trying to compare your body plan with a sponge. You could try, but the mere fact that you were trying would reveal something more psychiatric than anatomical.

To properly compare ourselves with these primitive animals, we need some tools. Just as with heads and limbs, our history is written within our development from egg to adult. Embryos hold the clues to some of the profound mysteries of life. They also have the ability to derail my plans.

THE COMMON PLAN: COMPARING EMBRYOS

I entered graduate school to study fossil mammals and ended up three years later studying fish and amphibians for my dissertation. My fall from grace, if you want to call it that, happened when I started to look at embryos. We had a lot of embryos in the lab: salamander larvae, fish embryos, even fertilized chicken eggs. I’d routinely pop them under the microscope to see what was going on. The embryos of all the species looked like little whitish batches of cells, no more than an eighth of an inch long. It was exciting watching development progress; as the embryo got bigger, the yolk, its food supply, got smaller and smaller. By the time the yolk was gone, the embryo was usually big enough to hatch.

Watching the process of development brought about a huge intellectual transformation in me. From such simple embryonic beginnings—small blobs of cells—came wonderfully complex birds, frogs, and trout comprising trillions of cells arranged in just the right way. But there was more. The fish, amphibian, and chicken embryos were like nothing I had ever seen before in biology. They all looked generally alike. All of them had a head with gill arches. All of them had a little brain that began its development with three swellings. All of them had little limb buds. In fact, the limbs were to become my thesis, the focus of my next three years’ work. Here, in comparing how the skeleton developed in birds, salamanders, frogs, and turtles, I was finding that limbs as different as bird wings and frog legs looked very similar during their development. In seeing these embryos, I was seeing a common architecture. The species ended up looking different, but they started from a generally similar place. Looking at embryos, it almost seems that the differences among mammals, birds, amphibians, and fish simply pale in comparison with their fundamental similarities. Then I learned of the work of Karl Ernst von Baer.

In the 1800s, some natural philosophers looked to embryos to try to find the common plan for life on earth. Paramount among these observers was Karl Ernst von Baer. Born to a noble family, he initially trained to be a physician. His academic mentor suggested that he study chicken development and try to understand how chicken organs developed.

Unfortunately, von Baer could not afford incubators to work on chickens, nor could he afford many eggs. This was not very promising. Lucky for him, he had an affluent friend, Christian Pander, who could afford to do the experiments. As they looked at embryos, they found something fundamental:
all organs in the chicken can be traced to one of three layers of tissue in the developing embryo.
These three layers became known as the germ layers. They achieved almost legendary status, which they retain even to this day.

Pander’s three layers gave von Baer the means to ask important questions. Do all animals share this pattern? Are the hearts, lungs, and muscles of all animals derived from these layers? And, importantly, do the same layers develop into the same organs in different species?

Von Baer compared the three layers of Pander’s chicken embryos with everything else he could get his hands on: fish, reptiles, and mammals. Yes, every animal organ originated in one of these three layers. Significantly, the three layers formed the same structures in every species. Every heart of every species formed from the same layer. Another layer gave rise to every brain of every animal. And so on. No matter how different the species look as adults, as tiny embryos they all go through the same stages of development.

To fully appreciate the importance of this, we need to look again at our first three weeks after conception. At the moment of fertilization, major changes happen inside the egg—the genetic material of the sperm and egg fuses and the egg begins to divide. Ultimately, the cells form a ball. In humans, over about five days, the single-cell body divides four times, to produce a ball of sixteen cells. This ball of cells, known as a blastocyst, resembles a fluid-filled balloon. A thin spherical wall of cells surrounds some fluid in the center. At this “blastocyst stage” there still does not appear to be any body plan—there is no front and back, and certainly there are not yet any different organs or tissues. On about the sixth day after conception, the ball of cells attaches to its mother’s uterus and begins the process of connecting to it so that mother and embryo can join bloodstreams. There is still no evidence of the body plan. It is a far cry from this ball of cells to anything that you’d recognize as any mammal, reptile, or fish, much less a human.

If we are lucky, our ball of cells has implanted in our mother’s uterus. When a blastocyst implants in the wrong place—when there is an “ectopic implantation”—the results can be dangerous. About 96 percent of ectopic implantations happen in the uterine (or fallopian) tubes, near where conception happens. Sometimes mucus blocks the easy passage of the blastocyst to the uterus, causing it to implant improperly in the tubes. Ectopic pregnancy can cause various tissue ruptures if not caught in time. In really rare cases, the blastocyst is expelled into the mother’s body cavity, the space between her guts and body wall. In even rarer cases, these blastocysts will implant on the outside lining of the mother’s rectum or uterus and the fetus develops to full term! Although these fetuses can sometimes be delivered by an abdominal incision, such implantation is generally very dangerous because it increases the risk of maternal death by bleeding by a factor of 90, as compared with a normal implantation inside the uterus.