While it took over 3 billion years for single celled life to develop into the most basic forms of multicellular animals (worms for instance), the more complex forms of life that surrounds us today evolved in under 600 million years. How could this be? Why would it take billions of years to evolve a blob of bacteria, but only a few hundred million more to evolve a monkey? Much of the answer has to do with the evolutionary leap of sexual selection.
For most of Earth's history, organisms were asexual. To reproduce, cells would simply divide, cloning the former as a near exact copy. Any mutations within cells would be passed along incrementally to be tested immediately within their environments. But this process was slow, and many potentially helpful mutations were lost because they occurred in the wrong time or place. However, some bacteria developed the ability to exchange genetic material with each other. This sharing of DNA is thought to have functioned as a crude precursor in the evolution of sexual reproduction within species.
Sexual reproduction established a way to combine and mix genes between members of a population. This aided their evolution in a variety of ways. Individuals now carried genes from each parent, allowing them to use code from either if there was an adverse mutation in one. But more importantly, it allowed much larger variation within the population. Through the shuffling of genetic material from each parent, each offspring is unique. Traits can lay dormant for generations only to reappear when conditions are right. This diversity made species more resilient to the constant flux of nature, and it created a genetic mix that's difficult for parasites and viruses to target. Sexual selection was so effective that it is now an essential part of the majority of Earth's plants and animals.
While sexual selection required more energy to be expended in mating, the advantages it provided were well worth the cost. Through sexual reproduction, evolution became more efficient. Instead of having to depend on environmental pressures to slowly remove poorly adapted traits, now the very act of choosing a mate became another overlapping way to select for certain traits. Typically males of the species will compete for the opportunity to mate with the female, where the female tends to be more selective because of the energy involved in raising offspring. As this mating process became more competitive, certain traits became increasingly favored, raising the bar for future generations.
Sexual selection in species tends to favor those traits that act as an indicator of health, with more elaborate traits for those who are less susceptible to environmental pressures. For many species it's their strength, stature, and fighting ability. Within some of these species this sexual selection process is so rigid that only one dominant male will earn the right to mate with a harem of females, ensuring that only the most strongest genes are passed on. For salmon it's their endurance through the migration (concurrent natural & sexual selection). And for many birds who can more easily evade predators and so have little need to hide, it's their colorful plumage or song.
Sexual Selection's Effect Upon Human Behavior
Much of human behavior today can be still explained in terms of what evolution has encoded within our genes. This is particularly true in regards to love and sex. Our bodies and minds are wired for it, and as a consequence we spend much of our lives in pursuit of a mate.
The emotions associated with this drive are intense for a reason. Sexual attraction & pleasure evolved to give a strong incentive to reproduce, and so those traits became amplified through the generations. During mating the senses become heightened, the skin more sensitive to the touch. Adrenaline is released into the blood, increasing the heart rate. The body's resulting release of neurotransmitters surges like electricity through the entire nervous system. It's enough to remind us that we too are animals.
The behavioral differences between males and females largely stem from this process of sexual selection. The testosterone of males gives an incentive to compete with other males for mating opportunities. Within females, estrogen results in stronger emotional bonds, with a mate that can protect against possible threats, and towards offspring needing to be nurtured. And since females can be fertilized only once, they tend to be more selective in their partners than their male counterparts. This biological process also gave the sexes a tendency towards jealousy, driving each to protect their own self interests. The focus of which may relate to things representing security & stability, but it's typically felt strongest in relation to mating.
While most modern humans have moved beyond the full instinctual control of these carnal emotions, they still guide us nonetheless. They can provide for some of the richest experiences of love and passion that we humans are capable of, as well as a catalyst for heartbreak and stress. No amount of social-conditioning is going to replace this fundamental biology. But perhaps we can see these emotions as they truly are, and act upon them accordingly, if we know the purpose they serve.
"Analyse any human emotion, no matter how far it may be removed from the sphere of sex, and you are sure to discover somewhere the primal impulse, to which life owes its perpetuation."