The Peacock’s Tail: How Microbes Help Drive the Evolutionary Process, or Why women fall for jerks explained.


2 million years ago, or thereabouts, Homo erectus diverged from an earlier species of hominid known as Australopithecus. Evolved is not the correct word here, as both species coexisted, at least for a time. Something had happened to one group of these hominids that forced an evolutionary adaptation. The adaptation generated a new species. But how?

Malarial diseases have been a part of hominid, and human, evolution for millions of years. In fact, malaria has evolved right along with humanity, often decimating entire populations. However, sometimes, on rare occasions, infectious diseases get greedy. One modern example is rabies, which left untreated, kills 100% of its victims. Around 300,000 years ago, an incredibly aggressive strain of a disease appeared. It I considered by medical anthropologists to have been a form of malaria.

The parasite that causes malaria, Plasmodium, survives best if it does not kill its host. But this was a very aggressive strain. Humans and other Great Apes carry a gene that codes for a molecule called Sialic Acid. Sialic acid is a type of sugar found on the surface of cells. They play an important role in cell-to-cell communication, and they serve as binding sites for receptors. Through the evolutionary process, some pathogens, including those that cause malaria, use these molecules to invade living cells. While the Plasmodium parasite benefited from this molecule by bonding to it. Not all humans possessed the gene that codes for Sialic acid. When this extremely aggressive strain of malaria struck, the result is easy to predict: individuals that lacked the gene for Sialic acid survived the epidemic. Those individuals who could produce the chemical did not. This did not happen overnight, but over a few hundred years, those individuals not possessing this gene thrived and passed along their genome to their offspring. Those with the Sialic acid producing gene died out.

A similar series of events took place when modern humans migrated out of Africa about 100,000 years ago. Interestingly enough, these modern-appearing humans coexisted for a time with the far more archaic homo Erectus previously mentioned. It was once thought that genetic drift and random mutation with the principle driving forces of evolution. While it was considered that climate change, scarcity of certain foods, and even migration patterns played key parts (as Darwin discovered on the Galapagos Islands when cataloging the variety of Finches). While the biological processes used by evolution is based in large part on genetic mutation, microbes including viruses and even some parasites are now understood to have played key roles in defining and redefining species. While the common understanding is that microbes are bad, particularly bacteria and viruses, the reality is much different.

Let’s look at an infection of an opportunistic bacterium or virus. Much of your immune response to this invader is aided by microbes that have taken up residence in our bodies.

Before an invading microbe can infect us with a disease, it must face microbes that have become native to our internal environment, and these microbes help defeat invading pathogens. Not to suggest that this has been done consciously by either species. For most of man’s existence, either in one form or another, little concern has been warranted in avoiding illness (many would argue that collectively that humankind still lacks the basic understanding of Health and Wellness). The flora and fauna that live within our systems have migrated there over countless eons, and most probably with little conscious effort. The great scientist and writer Dr. Lynn Margolis coined the term “symbiosis” to describe’ interactions between two different organisms living in close physical association, typically to the advantage of both.

Margolis discovered that within living cells, the mitochondria had originally been a separate life form. However, often when bacteria or viruses invade our bodies, symbiosis is not possible. The invaders tax our systems too much or are identified by the friendly bacteria in our systems and the battle ensues. The bacterium that had invaded our systems gave little consideration to its actions, it merely tries to replicate and survive. A good example of a microbe that has caused untold human suffering is an innocuous little microbe called Vibrio cholera.

Vibrio cholera, in its natural state, is a sea-dwelling microbe, often found living on, or in, tiny crustaceans called copepods. Happily floating about in the brackish water, the tiny Vibrio has no interest in infecting humans. In fact, it would take human engineering to bring these two species, Vibrio, and human, together.

In India, the Ganges Delta was in part filled in to allow the over-crowded villages and farms to expand. Villages and farms require copious amounts of freshwater, yet the closest source was the brackish waters of the delta. The water was, of course, undrinkable, so the villagers sunk wells. In retrospect, it was inevitable of course, that the Delta water would seep into the ground water, and then into the wells, along with the copepods and their Vibrio hangers-on. Humans had never made it a practice to drink brackish water, and so never encountered the copepods or the vibrio in any significant numbers. With the high levels and fierce waves driven by the winds of the monsoon seasons, the new ground-water wells were inundated with water from the delta; once humans began ingesting the vibrio in numbers, infection followed.

So what does this have to do with the Peacock’s tail?

In the same geographical area as Cholera first emerged, the Indian Peafowl makes its home. The male of the species, the Peacock (as opposed to the Peahen) has an enormous, brightly colored and showy tail plumage. Zoologists have remarked that the tail feathers are so large in fact that they are detrimental to the bird’s ability to fly. And yet, evolution has driven the length of the Peacock’s Tail, as longer, more colorful tails appeal most to the Peahens. But here’s the interesting thing: biologists have determined that Peacocks with larger tails, also possess higher production of B-cells, and, the size of the eye spots to T-cell production. The Peahens then were, unknowingly, selecting males with the strongest disease-fighting immune systems to pass along to their offspring. And it did not stop with birds. Modern humans are also unconsciously governed by their desire for healthy offspring with healthy immune systems.

For Homo Sapiens, or Wise, Wise Man (arguably, social behavior over the past few hundred years would call into question how wise we are), a similar phenomenon is occurring. Human males who have facial features women find most attractive (thin lips, wide and well defined jaw, heavier brow ridges, larger noses, less prominent cheek bones) were positively associated with sexual arousal. The two authors who conducted this study wanted to examine how immune function, testosterone, cortisol (a stress hormone), was associated with facial attractiveness. Seventy-four men were checked for blood levels of cortisol and testosterone, and then gave them a vaccine. The vaccine causes an immune response; the body produces antibodies, which will then attack if the person is infected with the particular disease (the amount of antibody produced after you receive the vaccine is a measure of immune function).

They compared these men to those that women had rated as most attractive. The findings were interesting to say the least. Males with higher levels of testosterone had statistically higher levels of antibodies after vaccination; they were the same males that female judges had identified as being more attractive. So the researchers found that women were more attracted to men with higher testosterone levels, different bacteriological makeup of their skin ( so they smelled different), and also had significantly higher immune responses. Marriage and family therapists, social psychologists, and common sense tell us that over-aggressive men tend to be poor choices for mates. So did you fall in love with the features, the smell, or was it the immune system that attracted you?

Now, when I was a young man, I asked a family friend why women seem to go for the “jerks.” He looked at me and said “If you can figure that out, you’ll get a Nobel prize.”

Nobel Committee? I am waiting….



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