Empathy is unusual in the animal kingdom, so empathy must have had some major survival benefits for it to have evolved. What might those benefits have been?
Empathy seems to have evolved in three major steps.
First, among vertebrates, birds and mammals developed ways of rearing their young, as well as forms of pair bonding -- sometimes for life. This is very different from the pattern among fish and reptile species, most of which make their way in life alone. Pair bonding and rearing of young organisms increased their survival and was consequently selected for, driving the development of new mental capacities. As neuroscientists put it, the "computational requirements" of tuning in to the signals of newborn little creatures, and of operating as a couple, helped drive the enlargement of the brain over millions of years. As we all know, when you are in a relationship with someone, and especially if you are raising a family together, there's a lot you have to take into account, negotiate, arrange, anticipate, etc. No wonder brains got bigger. (It may be a source of satisfaction to some that monogamous species typically have the largest brains in proportion to bodyweight!)
Second, building on this initial jump in brain size, among primate species, the larger and more complex the social group was, the bigger the brain was. (And the key word here is "social," because group size alone doesn't create a big brain; if it did, cattle would be geniuses.) In other words, the "computational requirements" of dealing with lots of individuals -- the alliances, the adversaries, all the politics! -- in a primate band also pushed the evolution of the brain.
Third, living in small bands in harsh conditions in Africa and breeding mainly within their own bands, our hominin and early human ancestors were under intense evolutionary pressures to develop strong teamwork as a band while they competed fiercely -- and often lethally -- with other bands for scarce resources. Hominins starting making stone tools about 2.5 million years ago, and during the 100,000 generations since, the brain has tripled in size; much of that new neural volume is used for interpersonal capacities, such as empathy, language, cooperative planning, altruism, parent-child attachment, social cognition and the construction of the personal self in relationships.
In sum: More than learning how to use tools, more than being successful at violence, more than adapting to moving out of the forest into the grasslands of Africa, it was the complexities of relationships that drove human evolution!
Homo sapiens means "clever ape." We are clever, to be sure, but we are clever in order to relate. It would be perhaps more accurate to call our species Homo sociabilis, the sociable ape.
As Charles Darwin wrote, "All sentient beings developed through natural selection in such a way that pleasant sensations serve as their guide, and especially the pleasure derived from sociability and from loving our families." Sociability, and the empathy at the heart of it, drove evolution; in a fundamental sense, it is empathy that has enabled this blog to be posted by me and read by you.
Empathy is in our bones. For example, infants will cry at the tape-recorded sound of other infants crying, but not at a recording of their own cries. And speaking of crying, as adults, our tear glands will automatically start producing tears when we hear the crying of others, even if we have no sense of tearing up ourselves.
Perhaps an even better name for ourselves would be Homo empathicus.