Probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed. . . . [W]hen we regard every production of nature as one which has had a history . . . every complex structure and instinct as the summing up of many contrivances, each useful to the possessor . . . how far more interesting . . . will the study of nature become! . . .

In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation.

—Charles Darwin, The Origin of Species

To get to the roots of human nature, let’s begin, well, at the beginning. Planet Earth arrived on the galactic scene roughly 4.6 billion years ago. Within a billion years after that, conditions were ripe for the appearance of a completely unique and unprecedented form of matter—LIFE! To stay alive, these fledgling single-celled creatures had to have the capacity and motivation to approach that which was good for them and avoid that which was bad for them. Given that every individual life form was of finite duration, to keep life “breathing” (to borrow Darwin’s term) over time, there had to be a way for these early pilgrims to reproduce. Primeval reproduction was by fission, a simple process of splitting that resulted in offspring identical to the parental cells. How could these very simple original life forms lead to the fantastic variety of creatures that have and continue to inhabit our planet? Evolution, the idea that species change over time and are descended from common ancestors, goes back at least to the ancient Greek philosopher Anaximander (610–546 bc). Charles Darwin’s genius was to propose the theory of natural selection to explain the process through which evolution occurs.

Perfectly self-replicating life forms never could evolve; they would continue to produce exact copies of themselves as offspring. But nothing about life is perfect, so mistakes in this most basic reproduction process sometimes occurred, and slightly different variations of the “primal creature” resulted. Some single-cell protozoans began to reproduce by fusion in addition to fission; for example, paramecia lying close to each other exchanged genetic material before dividing into multiple new organisms. This process eventually led to the prevalence of species that rely on sex for reproduction. These processes of reproduction cause variability, the first ingredient in the recipe for evolution by natural selection. There are two primary sources of such variability:

The second key ingredient for evolution by natural selection is competition. In a world of limited food, mating partners, and other resources, even infinitesimally small variations might help an individual compete more effectively, at first with other “primal creatures” and eventually with other species vying for the same resources in the same environments.

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Variability and competition in particular environments determine which genes are passed along to subsequent generations through reproduction. These genes influence each individual organism’s physical and behavioral attributes. Those creatures that possess attributes that improve their prospects for survival, reproduction, and survival of their offspring are more successful in passing along their genes, which, in turn, leads to the widespread representation of those attributes in future generations. These attributes are known as adaptations. Over time, individuals with the most successful adaptations outnumber and eventually replace less well-adapted versions of the creature.

The process of evolution by natural selection has often been characterized as the survival of the fittest. (Darwin himself never used this term; it was coined by Darwin’s contemporary Herbert Spencer in 1864.) However, “fittest” does not mean strongest or most aggressive. If it did, Tyrannosaurus rex would still be roaming the planet instead of merely posing in fossilized form for museum patrons. What is “fittest” depends entirely on the natural environments in which particular organisms reside. Creatures adapted for warmth would not be fit in cold environments, and vice versa, which is why you don’t find alligators and iguanas in Alaska or polar bears and penguins in Panama (except in zoos!).

Once new variants on a life form emerge, with new ways of exploiting an environmental niche for survival and reproduction, attributes that were once adaptive may become less adaptive. If these less useful attributes aren’t harmful, they may remain as harmless vestiges of early ancestors. Other attributes that might have been utterly useless to past generations may now acquire tremendous adaptive value. For example, if mutations cause individuals of an aquatic species to develop new ways of obtaining oxygen so that they can survive on dry land, what was functional for their ancestors (e.g., gills) may become utterly worthless. However, new variations—such as body protuberances that make it possible to move around on land (rudimentary legs)—may now become especially advantageous. So the process of evolution is highly dependent on changes in both the external environment and the characteristics of the organisms in the local environment.

Depictions of evolution in popular culture often describe “Mother Nature” as calling the shots, encouraging one species to realize its full potential while neglecting or punishing another species. But this is false. The process of natural selection just happens—it has no intention and strives toward no goal. Rather, variability and competition are simply facts of life, and so organisms evolve by a gradual refinement of previous adaptations. A fairly random mixture of characteristics of environments and organisms determines which attributes are adaptive and which are maladaptive. For example, bipedalism (standing up on two legs) gave early humans significant benefits, such as freeing up their hands for using tools, but it has its downsides, including slipped disks, fallen arches, and shin splints. Like much of life, evolution is a series of trade-offs where even those attributes that were beneficial enough to be passed down over generations can also come with certain costs. Keeping this in mind, we’ll want to avoid making the naturalistic fallacy, whereby we assume (quite incorrectly) that the way things are is necessarily how they ought to be (Ismail et al., 2012).

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