She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity (44 page)

It's even possible to
manipulate people's genetic essentialism. In 2014, Dar-Nimrod and his colleagues had 162 college students fill out questionnaires about the foods they liked and their eating habits. Then they all read what looked like a newspaper article, which in fact had been written by the scientists. Some students read an article explaining that obesity is caused by bad genes. Others read an article explaining how people get obese if they're surrounded by friends who eat too much. Another group read an article about food with no mention of obesity. Finally, all the students were led to another room, where there was a big bowl of chocolate chip cookies that were broken into pieces.

The scientists explained they were going to use cookies in another experiment but they wanted to make sure they tasted right. They asked the students to eat some cookies and give their opinion. In fact, the cookies were part of the experiment, too. After the students left, the scientists measured how much they had eaten. The students who had read the genetic article ate almost 52 grams of cookies. The ones who read the article on
social networks ate only 33 grams, and the ones who read the article that didn't mention obesity ate 37 grams.

The concept of genes driving people's appetite caused them to lose some control of their own. In a society that practically worships DNA, we are running this experiment on a colossal
scale.

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like a split screen. On the left, picture a single bacterium. On the right, a fertilized human egg.

The bacterium grows, duplicates its DNA, and splits itself in two, then four, then eight. The eight bacteria are kin, joined to the original microbe by the bonds of heredity. They inherited copies of its chromosome. Each new generation of bacteria is made of the proteins, RNA molecules, and other ingredients of their mother cell. You could trace their heredity over time by drawing a branching family tree.

The fertilized egg does much the same thing. It grows, copies its DNA, and splits in two, then four, then eight. The human cells may be hundreds of times bigger than the bacteria, and they may divide at a far slower pace. But in a developing embryo, cells share a heredity as well. Each pair of new daughter cells inherits copies of their mother cell's DNA, along with half of its other molecules.

It may seem strange to use the language of heredity for what happens in our own bodies. We tend to think of heredity only as a way to link ourselves biologically to the past and to the future. Yet heredity does not stop when a new life begins. Each of the 37 trillion cells in our bodies resides on a branch of a genealogical tree that runs all the way back to our origin at conception.

Before long, the split screen diverges. As the bacteria divide, they grow into a jumbled colony of identical cells. The egg's descendants, on the other
hand, develop into a human form, complete with a head and face, with fingers and toes. And along the way, the cells that make up the new embryo give rise to different kinds of cells. Now its inner heredity shifts to a new style. Each cell in the stomach lining gives rise to two cells of the stomach lining. Now bone cells divide reliably to make more bone rather than pads of fat.

Textbooks say that the human body has about two hundred cell types, but recent studies have rendered that figure a laughable understatement. No one can say
how many cell types there are, because the more scientists examine cells, the more they break down into more types. Immune cells may all carry out the same mission to save us from pathogens and cancer, but they are an army with hundreds of divisions. All our cell types are separate branches on the body's genealogical tree, like rival dynasties descended from a first monarch.

This transformation poses the chief question that developmental biologists have asked for centuries and continue to ask: How does a cell with a single set of genes give rise to the complexity of the human body? Heredity supplies the answer, but it does so in different forms. Heredity, in other words, is more than one thing.

Aristotle asked himself this question, but he could try to answer it only by cracking open chicken eggs. If he opened an egg the day it was laid by a hen, he saw only the white and the yolk. He observed nothing more on the second day, or the third. But on the fourth day, he could make out a red speck. This he took for the heart. “
This point beats and moves as though endowed with life,” Aristotle said.

Over the next few days, other things became visible in the egg. Tubes filled with blood sent out branches. The dim mass of a body emerged. Aristotle could eventually make out a head, adorned with a pair of bulging eyes. A chicken embryo at this stage looked a lot like the embryos of other animals Aristotle had studied. But as the days passed, the similarity faded, overtaken by the peculiar features of birds—a beak, feathers, claws, wings. “About the
twentieth day, if you open the egg and touch the chick, it moves inside and chirps,” Aristotle observed.

Some philosophers of Aristotle's age believed that the parts of a chicken, or any other animal, somehow existed in miniature even before this development. Aristotle would have nothing of it. He saw the development of an embryo as akin to making cheese. A cheese maker added fig juice to milk, setting about a transformation that created something that did not exist beforehand. When chickens mated, the rooster's semen triggered a similar transformation of fluids inside the hen. Organs curdled into existence in an unfolding sequence. The spirit within the semen shaped a heart, which in turn shaped other organs, which in turn shaped more of the chick's body until it was complete.

For two thousand years, Western scholars and physicians hewed closely to Aristotle's vision, but the Scientific Revolution brought a realization that he had gotten some things wrong. William Harvey, the royal physician to King James I and Charles I of England, searched for Aristotle's curdling fluids inside does and hens. He could find none. Like Aristotle, Harvey inspected embryos of chickens. The heart did not form first, he realized: Blood vessels took shape earlier. To account for these differences, Harvey came up with a different vision of life's beginning: It was from eggs that all animals grew. When he published a book about his idea in 1651, he emblazoned it with the Latin motto
Ex Ovo Omnia
.

You can search Harvey's book from cover to cover and still be left baffled by what he meant by
ovo
. While Harvey was sure that mammals had eggs, he could never find any evidence of them. Instead, he speculated that the hypothetical eggs were produced by the female body
much as the mind produces thoughts. Semen then acted on the eggs so that they began to develop into embryos.

In this regard, Harvey still remained faithful to his hero Aristotle. The different parts of the body all emerged from a homogenous beginning. He called this unfolding
epigenesis.

Other scholars in the seventeenth century began promoting a radically different theory for how new generations emerged. Known as
preformationists, they argued that all the anatomy of an animal already existed before conception. In the 1670s, a Dutch naturalist named Nicolaas Hartsoeker discovered sperm, using newly invented microscopes.
He drew the head of a sperm with a tiny human lodged inside.

Preformationism held sway until the mid-1700s, when new observations exposed its flaws. A German medical student named
Caspar Friedrich Wolff studied chicken embryos more carefully than anyone ever before and could find no trace of a miniature bird in their earliest stages. Instead, he saw a blob of unorganized tissue that gradually took on new structures, which only later turned into recognizable parts of a chicken's body.

The 1800s brought more powerful microscopes, which scientists used to make new discoveries about development. Only then, for example, did they finally discover the mammal eggs that Harvey had dreamed up two centuries earlier—first in a dog, and then in a woman. After fertilization with sperm, eggs could then develop. It also became clearer what the eggs were developing into. Under the new microscopes, animal bodies resolved into tiny units. These units looked different depending on what tissue they came from: the blobs in blood, the long fibers in muscles, the brickwork of skin. But researchers realized
they were all variations on a theme. “There is one universal principle of development for the elementary parts of organisms,” the German zoologist Theodor Schwann declared in 1839. “
And this principle is in the formation of cells.”

The new cell theory raised new questions of its own, such as how cells came to be. Some naturalists argued that cells spontaneously formed out of biological fluids, the way hard-edged crystals could form out of a featureless soup of chemicals. But a group of German biologists proved that new cells emerged only from old ones. It was a microscopic form of like engendering like. The biologist Rudolf Virchow decided it was time to update Harvey's motto.
Ex ovo omnia
became
Omnis cellula e cellula—
every cell comes from a cell, inheriting its traits from its ancestor.

Cells, it became clear, were not unique to animals. Plants were made of cells, as were fungi. Bacteria and protozoans had bodies made up of a single cell. Different forms of life made new cells in different ways. Bacteria,
for example, simply divided in two. A species like yeast used a somewhat different cycle. A mother cell might split in two, but the two daughter cells would remain firmly stuck together. As the division continued, the yeast grew into a mat—not exactly a body like ours, but not a group of isolated organisms either. Animals and plants, on the other hand, developed into giant collectives of cells, which reproduced by making new collectives.

To reproduce, some aquatic animals such as corals and sponges simply break off a packet of cells known as a bud. The bud drifts away, settles on a new spot on the seafloor, and grows into its own full-size body. When animals reproduce by budding, it's hard to draw a clean dividing line between ancestors and descendants. They all belong to the same unbroken line of cell divisions. Even though the new animals have distinct bodies, you can still think of them as overgrowths of their ancestors.

Most animals, humans included, can't reproduce by budding. Cut off your arm, and it will not grow into a second you. We and most other animal species develop from a single fertilized egg, called a zygote. Like other cells, zygotes don't leap into existence out of the void. Each is the fusion of two older cells. The continuity of zygotes with the previous generation led some scientists to propose that children are
the overgrowth of their two parents.

Overgrowth
sounds like a chaotic, weedy explosion of life. But the development of animal embryos is nothing of the sort. Most animal embryos change from a nondescript ball into a shell, with a clump of cells stuck to its interior wall. The cells making up the shell become the placenta, while the clump becomes the embryo itself. The clump spreads out into a sheet made up of three layers. Those layers came to be known as the ectoderm, the endoderm, and the mesoderm. You started out from these three layers, and so did a grasshopper, and so did a tapeworm. Those layers go on to form different tissues of the body.

When biologists began to look at later stages of developing embryos, they could start making out newly formed tissues. Each type was made up of its own distinctive set of cells. Yet no matter how distinctive they became on the outside, they still remained similar within. A sprawling neuron and
a sheet-shaped epithelial cell both had a nucleus at their core, inside of which they held identical chromosomes.

To August Weismann, the man who cut off mouse tails to champion Darwin over Lamarck, this unfolding variety was hard to fathom. “
How is it,” he asked, “that such a single cell can reproduce the
tout ensemble”—
the total impression—“of the parent with all the faithfulness of a portrait?”

Weismann's many years of looking at animal embryos led him to an answer. When a fertilized egg divided, it bequeathed its nucleus to its offspring. And inside that nucleus was
a mysterious thing Weismann called “hereditary tendencies.” Those descendant cells passed down the same tendencies when they divided. Weismann reasoned that the only way that cells in an embryo could take on different identities would be to inherit different hereditary tendencies.

When a cell divided, in other words, it had to determine which of its daughters inherited which tendencies. Early on in development, a cell might bequeath the tendency to become ectoderm to one cell, and the tendency to become mesoderm to the other. Each cell could then pass down only its specific tendency to its daughter cells. Later on, an ectodermal cell might divide its hereditary tendencies unequally once more. Some of its descendants might inherit only the tendencies for becoming a skin cell, others a nerve.

For Weismann, in other words, the development of an embryo was a saga of loss. By the time organs like stomachs and thyroid glands emerged, their cells lacked most of the original hereditary tendencies in the fertilized egg. They could only divide into more stomach cells or thyroid cells. They could never produce a new animal,
tout ensemble
.

Thinking about development this way, Weismann turned his attention to how embryos produced their own supply of new eggs or sperm. He had observed this process for himself, and he had been struck by how early they developed, and how they then were set aside while the rest of the embryo continued to grow. Weismann became convinced that this early isolation was vital, because eggs and sperm had to be set aside before they lost too many hereditary tendencies. There must be a profound distinction between
sperm and eggs, which Weismann called germ cells, and the rest of the body, which he called somatic cells.

Weismann split heredity in two. One form of heredity joined parents to their children. According to Weismann, parents were custodians of the germ-plasm, a mysterious hereditary substance that could produce an entire human being. Over the generations, the germ-plasm never lost its ability to give rise to new life.

Germ-plasm heredity gave geneticists the concept they needed to make sense of Mendel's experiments, to observe how hereditary factors could hop down through the generations like rocks skipping across a pond. To geneticists, what happened during development didn't matter much. It was just a dead end made of disposable flesh.

But
Weismann also recognized another kind of heredity playing out inside of each of us. He illustrated this inner heredity with pictures. In his book
The Germ-Plasm: A Theory of Heredity
, Weismann represented a threadworm's development in the form of a tree—an embyronic pedigree, as it were. At the base of the tree he drew a circle, representing a single fertilized egg. The circle sprouted a pair of branches, to stand for the zygote's division into two daughter cells. One branch led to a white dot, which branched in turn into more white dots. These represented ectodermal cells. The other branch gave rise to other lineages—of endoderm, mesoderm, and germ cells. If you didn't know you were looking at a threadworm, you might think you were looking at the Habsburg dynasty's family tree.