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Although alien invasion of the body most often poses a threat, sometimes it is the key to perpetuation of the species. Human semen, as much an outsider as any bacterium, enters a body to fertilize, leaving behind an object that grows rapidly in the womb. Since the immune system would quickly destroy sperm and fetus if it found them, the body has evolved ways to keep its army in the dark.

Sperm arrive coated with a protective substance that conceals their antigens from the female’s phagocyte patrols. The coating also seems to offer protection against the activity of female T and B cells. The mother’s body collaborates on this, applying a second coat that further masks the sperm’s intolerable antigens.

The sperm cell’s mission is a short one, its exposure brief. But the fertilized egg faces a more persistent danger. Each cell of the fetus wears the insignia of the mother and father, making it legal game for the mother’s immune system, which travels in the same blood with which the fetus must be nurtured in the womb.

Connected to the mother through an umbilical cord, the baby is shielded by a sophisticated filter in the placenta, the branching of the umbilicus where it attaches to the wall of the womb. Although the bloodstreams of mother and child converge here, the do not quite meet. Oxygen and nutrients from the parent’s blood are filtered through an inner lining of the placenta called the trophoblast, which removes or chemically disguises fetal antigens that the mother’s immune system would otherwise attack. The trophoblast has other stealth tactics as well, launching decoy antigens into the farther reaches of the womb to draw the attention of killer cells. At the same time, it admits antibodies against certain bacterial enemies, passing the immunity of the mother on to the unborn child.

If this first line of defense fails, the fetus is far from powerless. It can produce substances that encourage the mother to generate suppressor T cells, which turn off her immune response.

Ironically, the defensive strategies of human beings at the beginning of life resemble those employed by a less welcome intruder. It appears that some cancer cells, like human fetuses, somehow persuade the host’s immune system to call a false truce.

One theory of human evolution holds that we never quite grew up. Certainly, such characteristics as curiosity and innovation can be considered hangovers from childhood. But there may be reason to think that we are childlike in other, more tangible ways.

In 1926, Dutch anatomist Ludwig Bolk noted striking similarities between humans and chimpanzees before and just after birth. The embryonic chimp is hairless, with a flat face, no bony brow ridges, a rounded head, and small teeth; once born, it continues to bear a striking resemblance to human infants (below). But in the chimp and other primates, these similarities quickly mature into the familiar low-browed, jutting-jaw look of the ape. In humans, however, the same infantile features persist into adulthood, often along with the playful behavior seen in all primate juveniles. In a sense, humans never stop being young.

Besides retaining childlike features and behavioral traits, humans are notoriously slow in their early development compared to various animals. Nine months after conception, human babies come into the world as the most helpless of primate offspring.

According to some researchers, such helplessness suggests that the real period of human gestation is more like twenty-one months. In this view, all babies are born prematurely so that their bodies---their rapidly growing heads especially---can pass through the mother’s birth canal. Thus, human newborns may be considered as spending their first year as out-of-the-womb fetuses, vulnerably continuing their gestation even as they begin to learn about the world and its challenges.

From earliest times, no doubt, humans pondered the mysteries of birth---not least the question of how babies developed in the womb. One possible answer to the developmental puzzle was that the fetus began as a tiny homunculus, or little human, somehow created by the union of man and woman. This belief persisted even into the era of microscopy. The homunculus, some scholars said, was encapsulated in the head of a sperm cell. (The mother’s ova, or eggs, were intended merely to nourish the tiny traveler within.) Others insisted that the full-featured little human was latent in the ova but animated by the arrival of sperm.

Not until the mid-eighteenth century did the illogic of preformation become apparent. If all humans were preformed, each tiny homunculus must contain the homunculi of its offspring, and these the homunculi of theirs, and so on---people within people, like an infinity of nested Russian dolls.

In the nineteenth century, the concept of human evolution and a much more detailed knowledge of the human embryo’s stages of development coalesced into something more like scientific theory. Germany’s pioneer embryologist Karl Ernst von Baer noted in 1828 that the embryos of many species are similar in their early stages but rapidly diverge thereafter. German biologist Ernst Haeckel took the notion a step further. Stimulated in his thinking by the theory of evolution that was published by the Englishman Charles Darwin in 1859, Haeckel proposed that the process of evolution is repeated by the embryo as it grows in the womb. Haeckel called his idea the biogenetic law.

Like the idea of the homunculus, however, Haeckel’s “law” rested on an illusion. Embryos of such vastly different creatures as fish, birds, reptiles, and human beings may seem almost identical, perhaps reflecting a common ancestor at the dawn of life. But this resemblance, it turns out, is superficial at best and persists for only a short interval. Early in their gestation, various embryos rapidly diverge along their separate genetic paths to become the vastly different creatures that emerge at the time of birth.

In its vast inventory of parts, the body includes some that appear to have lost their original function or that seem to serve no purpose whatsoever. Because they often resemble the organs and adaptations of lower and more primitive life forms, these spares are taken to be vestiges of earlier stages of human evolution.

In the human embryo, for example, nonfunctioning gill structures, a two-chambered heart, and three pairs of kidneys evoke a fishlike ancestor. Such evolutionary leftovers are usually transformed in the womb as gestation proceeds. The gill structures, for example, become components of the jaw, ears, and throat. But other anomalies survive whole. A number of embryologists say that the body contains more than a hundred of these, only a few of which are readily apparent.

Best-known is the vermiform (worm-shaped) appendix - the trouble-prone remains of the cecum, an intestinal structure that our plant-eating ancestors needed to digest large quantities of cellulose. Less well-known is the nictating membrane, which acts as a kind of diving mask in amphibians and an eye-wiping third eyelid in such birds as owls, but exists as a small fold of pink tissue in the corner of each eye. Muscles that animals use to aim their ears for better hearing can enable us to wiggle our own ears. Others muscles that pucker our skin into goose bumps are hangovers from a time when, frightened or angry, a hairy ancestor raised his hackles.

Tails, employed to such effect by many creatures, recur in human embryos ---at one preliminary stage the tail is one-sixth of the fetus’ length. It soon stops growing, turns inward, and by the fourth month fuses into the coccyx, which is located at the base of the spine. Evolution may have erased a visible human tail, but the appendage now and then reappears in some births as a kind of throwback to our early primate past. It is usually boneless, slightly prehensile, and only an inch or so long, although one measuring nine inches has been recorded.