Passage IV

NATURAL SCIENCE: This passage is adapted from Chance in the House of Fate: A Natural History of Heredity by Jennifer Ackerman (@2001 by Jennifer Ackerman).

I once chanced on a collection of brilliantly colored paintings of South American insects, some of them nightmarish—a giant cockroach scaling a pineapple, for example—but most of them showing the metamorphosis of moths and butterflies from egg to winged insect, the life stages presented as if they were happening all at once.

The paintings were the work of Maria Sybilla Merian, a naturalist and artist who left her home in Amsterdam in 1699 at the age of fifty-two and sailed to South America to paint the life cycles of insects in the jungles of Suriname. The journey would have been remarkable in any century, but was especially so in the late 1600s, when savants were still poking for answers to nature’s riddles in the ruins of the classical world, quoting the authority of Pliny, the natural philosopher who had confidently proposed that butterflies were born from dew, and drawing their knowledge of animals from medieval bestiaries. Birds were categorized 20 according to notions of nobility, from most noble (eagles and hawks), to wise (owls), to big (ostriches), and on down the line. Caterpillars, if they were considered at all, were classified apart from winged insects and lumped together with worms and serpents.

From the Suriname capital, Paramaribo, Merian sailed up the Suriname River, stopping along its banks to collect caterpillars, then watching them closely for signs of metamorphosis and painting them at the pivotal moment. Two years later, sick of heat and fever, she returned home, loaded with specimens and hundreds of paintings of iguanas and geckos, fighting snakes and frog-eating scorpions, and dozens of moths and butterflies in various life stages on their native cassava, guava, batata, and pawpaw plants.

If we are to believe her journal. Merian knew the details of metamorphosis perhaps more intimately than the savants. Her paintings and journals address the step-by-step transfiguration not just of Lepidoptera (butterflies and moths) but also of amphibians. In 1686 she had precociously noted the developmental odyssey of frogs from black grains that “fed on the white slime that surrounded them,” to tiny creatures that grew little tails so that they could swim, then eyes, then, eight days later, two little feet “from the skin at the back and after a further eight days two little feet at the front ... [like] small crocodiles. Thereafter the tail rotted away and they became proper frogs and jumped onto the land.”

Just what was going on in this rotting away wasn’t discovered until 1842, not long after the discovery of animal cells. That year a German biologist, Carl Vogt, peered through a microscope at a developing midwife toad, Alytes obstetricans. Vogt saw that the individual cells of the toad’s notochord were being “resorbed,” as he put it, swallowed up or sucked in. But he didn’t think his observations important, and they slipped into oblivion.

Twenty years later, scientists observing metamorphosis in flies, ants, and beetles noted in the changing 60 muscles and glands the wholesale death of cells. Later studies of developing bones, muscles, and other tissues in mammals suggested that the shaping of ear, eye, nose, tongue, intestinal tract, and trachea involves some removal of excess cells. By the turn of the century it was clear: to have shapely toads, butterflies, babies, one must have cell death.

The idea that death would chaperone growth was not easy to accept. But at least the role of cell death in the miracle of development seemed of minor importance. Then, in the early 1970s, a team of Scottish pathologists examined different kinds of tissue and discovered that cells have two radically different ways of expiring. Those that die accidentally, by injury or poisoning, swell up and pop like balloons—a visually obvious process the team called necrosis. But those that die naturally during development do so by quiet, efficient, nearly invisible means. This way of dying (which the team called apoptosis from the Greek for “a falling”) may be inconspicuous, but it goes on all the time in every kind of tissue, in nearly every kind of animal.

Cell death is the night-side of growth. Cells die en masse at incredible speed, not just to sculpt our body structures before we are born, but throughout our life, about ten billion each day, to adjust cell numbers, eliminate the injured, and dispose of cells deemed obsolete. We don’t diminish in the face of such massive, incessant loss for the simple reason that new cells are born in trade for those that die. The body precisely balances 90 cell birth and cell death.