Mice, men and other mammals come from a common little ancestor, who lived 80 million years ago. Mice and men share virtually the same set of genes, with so far, almost every gene found in one species also identified in the other, in a closely related form. Among some 4,000 genes that have been studied, less than 10 are found in men but not in mice. Overall, the genomes of all mammals are comparably similar, including those related to reproduction and sexual behavior. Clearly, within the mammal family, small genetic changes can result in huge evolutionary jumps.
Mammals survived the great extinction of 66 million years ago, which put an end to the age of dinosaurs. In the vacuum left by the demise of dinosaurs, mammals came to dominate the animal world, through explosive mutations and evolutionary jumps that created bats as well as whales! Let’s remember that it took 3,000 million years for the unicellular life to evolve into the first sexual multicellular. Then, it took 1,000 million years to generate the first small mammal, who could carry her baby inside the womb and provide milk after giving birth. However, only 60 million years separate the first little mouse and the great Charles Darwin!
Darwin realized that natural selection alone was unable to account for certain types of non-survival adaptations. In Descent of Man, he disclosed the concept of sexual-selection (for reproduction) as an evolutionary struggle as critical as the natural-selection (for survival). Darwin identified sexual-selection as a struggle mainly between the males of each species for either attraction (e.g. birds) or possession (e.g. mammals) of the females. The result of a win or loss would not be an immediate death for the unsuccessful competitor, but few or no offspring (i.e. eventual death of his genetic line).
100 years later, Robin Dunbar discovered the great impact of social-selection. He found how quickly the primate brains evolved to cope with the large number of intra-group social relationships. Machiavellian intelligence evolved to outsmart, deceive and manipulate the social competitors. An endless arms-race required ever more sophisticated minds and skillful bodies, to understand and influence the minds of others, and to build tools to excel them.
Genetic selfishness as projected on the social group has been a dominant evolutionary force among mammals, and especially among the more social primates. Sexual-selection and social-selection have been the key features of mammalian evolution, which were superimposed on top of the slower natural-selection. Those created runaway positive-feedback loops which have been responsible for our explosive growth of body and brain power. Without them, mammals could not have generated such species variety and such growth in geological dominance, in such a short time.
Any winning genetic feature that can attract females (e.g. colorful feathers) or fight male rivals (bigger body and horns) has led to greater reproductive variance among males. In case of birds, attracting females is much more important, because they can simply fly away. But among mammals, males usually fight to monopolize females; therefore, body size and natural armament has been more important.
Under natural and sexual selections, the original little mammals developed larger bodies and brains, to such extent that their mostly solitary and underground lifestyle was no longer sustainable. Through monopolizing sex with a group of females, the first male-dominated “family” unit emerged among the larger mammals. Such extended families formed the evolutionary units for the social-selection.
Mammalian females always invest much more energy into producing offspring than males invest, which makes them the “limiting resource” for reproduction. The so called Bateman's principle demonstrates that males compete vigorously for that limiting-resource, by trying to monopolize sex with females. Therefore, variability in reproductive success is much greater among males than females, i.e. some males have been much more successful in passing their genes than others. This trend has led to remarkable genetic and behavioral differentiations between males and females too.
Consequently, male and female mammals have been socially and sexually “selected” for having distinctly different features. Primates, e.g. our closest chimpanzee relatives, are very protective of their mating rights. Male chimpanzees routinely patrol and guard their territory, against any encroachment by neighboring male chimps. Fights regularly break out between males of one unit against the other, over territory and resources, sometimes leading to full-scale assaults. However, in most instances females are treated differently and made exempt from that hostile behavior. For example, a female from unit A can roam into the territory of unit B, stay or return. Also, after a decisive battle, all males of the defeated unit would be killed or banished, but females are absorbed into the winning group.
Such social behavior has also led to sexual dimorphism (male, female dissimilarity) among primates. Among the dominantly polygynous primates, the male body mass is much larger than (up to twice) the female. Among humans, the difference is also significant, but less pronounced (about 20%). More importantly, among most mammals, many social and sexual advantages (body size and brain power) have been “selected”, which led to run away “positive feedback” loops of explosive differentiations.
A positive feedback loop works by taking a small change in one generation and causing a bigger change in the next generation. For example, consider that a bodily or brain advantage in an ancient male mammal could give him the opportunity to mate with 10 females, fathering 100 of advantaged offspring. Competition among those advantaged offspring could lead to only few dominant males monopolizing sex with females, and generating even more advantaged offspring of their own.
Such positive feedback loops could start with a limited initial-condition (little mammals of 66 million years ago) and under differing boundary-conditions (geography and climate) radiate towards vastly differentiated directions. In the vacuum left by the demise of dinosaurs, that is how the mammals adapted to fly and turned into bats, or learnt to swim and turned into whales. Rapidly and explosively, new higher-level orders emerged on Earth that evolved from a common lower-order, and adapted to live on a variety of other lower-orders; as bats forage on insects, whales gorge on planktons, and stars feed on hydrogen atoms.
Picture: Mammal hand variations.
Reference: How Sexual Choice Shaped the Evolution of Human Nature, by Geoffrey Miller.