sible to extend the time the developing embryo is kept inside the mother and thus make the newborn better prepared for independent survival. However, the placenta requires complex adaptations, such as the suppression of harmful immune interactions between mother and embryo, delivery of suitable nutrients and oxygen to the embryo, and the disposal of embryonic wastes. The mammalian placenta evolved more than 100 million years ago and proved a successful adaptation, contributing to the explosive diversification of placental mammals in the Old World and North America.
The placenta also has evolved in some fish groups, such as Poeciliopsis. Some Poeciliopsis species hatch eggs. The females supply the yolk in the egg, which furnishes nutrients to the developing embryo (as in chicken). Other Poeciliopsis species, however, have evolved a placenta through which the mother provides nutrients to the developing embryo. Molecular biology has made possible the reconstruction of the evolutionary history of Poeciliopsis species. A surprising result is that the placenta evolved independently three times in this fish group. The required complex adaptations accumulated in each case in <750,000 years (Reznick et al., 2002; Avise, 2006).
Natural selection produces combinations of genes that would seem highly improbable because natural selection proceeds stepwise over long periods of time. Consider the evolution of the eye in humans and other vertebrates. Perception of light, and later vision, were important for the survival and reproductive success of their ancestors, because sunlight is a predominant feature of the environment. Accordingly, natural selection favored genes and gene combinations that increased the functional efficiency of the eye. Such mutations gradually accumulated, eventually leading to the highly complex and efficient vertebrate eye.
How complex organs, such as the human eye, may arise stepwise from a very simple structure can be observed in living mollusks (Fig. 1.1). The mollusks (squids, clams, and snails) are a very ancient group of organisms, older than the vertebrates. Marine organisms have variable visual needs, depending on their lifestyle. Limpets have the simplest imaginable eye; just an eye spot consisting of a few pigmented cells with nerve fibers attached to them. Slit-shell mollusks have a slightly more advanced organ, consisting of some pigmented cells shaped as a cup, which allow these mollusks some perception of the direction of light. Nautilus, a group of open ocean mollusks that have remained virtually unchanged for millions of years, have an extended and nearly closed cup, with a pinhole opening but without a lens. Murex, a group of marine snails, have eyes with a primitive refractive lens protected by a layer of skin cells serving as cornea. Octopuses and squids have eyes just as complex as the human eye, with cornea, iris, refractive lens, retina, vitreous internal substance, optic nerve, and muscle.