• For animals, copulation is a dangerous luxury. Animals in the act of mating burn energy and neglect opportunities to obtain food.
• They are also vulnerable to predators who want to eat them or rivals who want to take over their territory.
• Copulation is something to be accomplished in the minimum time needed for fertilization.
• Sex is pleasurable, but evolution made it that way. If our species weren’t getting some evolutionary benefit from our sexual activities, the world would have been taken over by mutant humans who evolved not to enjoy sex.

• The answer must have to do with the cost of repair. Think again about car repairs. Suppose you buy an expensive car that you expect to last for a long time. It makes sense to invest in regular maintenance, which is cheaper than discarding your car and buying a new one every few years.
• In the same way, how much energy an animal “should” invest in biological repairs depends on the cost of the repairs, and how long the animal can expect to live with and without them.

• People from sunny Africa have black skin. So do people from other sunny areas, such as southern India and New Guinea. Skin gets paler as you move north or south away from the equator, until you reach northern Europe, where you find the palest skin of all.
• Obviously, dark skin evolved in people who were exposed to a lot of sunlight, because dark skin protects against sunburn.

• Darwin had noticed that many animals have features with no obvious survival value, such as the long, colorful tails of male peacocks and the dark, shaggy manes of male lions.
• These features do help animals get mates, however, either by attracting individuals of the opposite sex or by intimidating rivals. Their genes will get passed on, and those genes will spread through a population because of sexual selection, or mating preferences, not because of natural selection.

• Agriculture brought great increases in the amount of food we were able to store. This meant that more people could survive. But agriculture also brought disease, inequality between the sexes and between social classes, and the tyranny of powerful rulers.
• Agriculture began to appear only about ten thousand years ago.
• For most of our history, all humans had to be hunter-gatherers, living by hunting wild animals and gathering wild food plants.
• Why did almost all our hunter-gatherer ancestors adopt agriculture? They adopted it, of course, because it is an efficient way to get more food for less work.

• Nutrition is another difference. Farmers concentrate on crops such as rice and potatoes, which are high in carbohydrates.
• Hunter-gatherers are healthy and suffer from little disease. They enjoy a varied diet, and they do not experience the food shortages and famines that can happen to farmers who are dependent on just a few crops.
• t is almost unimaginable for Bushmen, who use 85 edible wild plants, to starve to death. In the 1840 s, however, about a million irish farmers and their families starved when a plant disease attacked potatoes, which they relied on as their staple crop and food.

Why would we do something that we know to be harmful or dangerous?

• They do so because our drugs of abuse are addictive—once you start, the chemistry of the drug affects the brain, making you want to continue.
• Ten thousand years ago we “displayed” by challenging a lion or a tribal enemy. Today we do it in other dangerous ways, such as driving fast or consuming dangerous drugs.

• It is the instinct to rely on handicap signals as a sign of strength. Whiskey and cigarette ads are cleverly directed at that old instinct, with their messages that smoking and drinking will bring status and make us attractive.
• Drug addicts and alcoholics not only lead shorter lives, but they also become less attractive, not more, to possible mates. They also lose the ability to care properly for children.

What would happen if we found alien —or it found us. Would we and the aliens sit down together for fascinating discussions? Would they share their advanced technology with us?

• Throughout history, human explorers who discovered technically less advanced humans responded by shooting them, devastating their populations with new diseases, and destroying or seizing their territory. Any advanced terrestrials who found us would surely treat us the same way.
• For all practical purposes, we’re alone in a crowded universe. Thank goodness!

• One cloud is the risk of a nuclear war that could destroy us all.
• The second cloud is the risk of an environmental collapse. One potential cause of such a collapse is the gradual extinction of most of the world’s species.
• Isn’t extinction a natural process? If so, why should we worry about the extinctions that are happening now? Yes, every species will eventually go extinct. But the current human-caused rate of extinction is much higher than the natural rate.