Copyright © 2007-2017 Russ Dewey
Evolutionary perspectives are widely accepted within psychology. However, people who work closely with genetics offer several cautions:
–Almost all behavior is the result of multiple genetic influences (i.e. most behavior is polygenic). Seldom is there "a gene" for some behavior.
–A single genetic change usually results in multiple effects. This is called pleiotropy. Seldom does a gene affect only one behavior.
–A behavior that was adaptive in the ancestral environment might not be adaptive in tomorrow's environment. To find out if a current trait is adaptive, we must wait until the future and look back.
What are a few complications in relating genes to behavior?
–A behavior that evolved to serve one function at time A may serve a different function at time B. Functions and purposes are identified by human observers, but evolution does not rely on labels. Existing behaviors and genes may serve new purposes, as circumstances change.
Evolutionary theorists (since the mid-20th Century) spoke of evolutionary opportunism: the use of existing structures (genes, body parts, behaviors) for new purposes. Evolution always works with what is already there, so it uses existing components as building blocks. These old parts can be put to new uses. This is common in evolution.
For example, ground-dwelling birds may use pre-flight movements in mating displays, even though members of the species no longer fly. Some time in the past, pre-flight movements were assimilated into the mating display.
How did this happen? The behavior was available in the male bird's behavioral repertoire. So it was used. Evidently some females were attracted by this way of fluttering the wings.
If it served as a lure for mating, the genes enabling that movement provided a new selective advantage. The same movements that previously stayed in the genome because they prepared a bird for flight would now would stay in the genome because they help attract mates.
A truly useless body part or behavior will not be maintained over evolutionary time. Changes such as mutations gradually erode its form. If there is no cost to the species from its disuse, it will become vestigial or unused and eventually disappear, like the legs of whales.
A behavior that comes under a new set of selective pressures or advantages is said to be emancipated from its original function. It does not disappear because it makes a positive selection (has a beneficial impact on differential reproduction). Therefore the old body part or behavior stays around, shaped up and in good form, but with a new function.
Why do some body parts and behaviors disappear, while others are maintained?
Because of opportunism and emancipation, one cannot assume the present function of a gene or behavior is the same as its ancestral function. For example, dreaming might serve one purpose in non-human animals, but it might serve entirely different purposes in humans, today.
As far as we know, all mammals have REM sleep, and probably all of them dream. We see cats and dogs act out their dreams during sleep. However, they do not have language to grasp and remember dreams. Humans do. In our species, the vivid nighttime experiences became known as visions, often interpreted as messages from ancestors or gods.
When dreams became treated as warnings or predictions of the future, this gave them a new function in human society entirely different from the metabolic functions of dreaming, whatever they might be. New selective pressures might have favored meaningful dreaming in humans.
For example, people who have accurate warning dreams, or creative planning dreams, might have an advantage. If meaningful dreams played a positive role in human differential reproduction, then they may have become emancipated from their original functions.
How might meaningful dreaming in humans be the result of opportunism?
This is just one example of a pattern widespread in evolution. Things change, and old structures can take on new meanings and purposes.
Here are some common mistakes to avoid, in discussing evolution.
–Evolution should not be reified or treated as a thing or a force of nature. Evolution is just a word used to label change over time. Therefore it is erroneous to speak of evolution making something happen.
That type of language is a sloppy shorthand for saying that some factor had consequences for reproduction in a species. "Evolution caused island dwarfism" is a quick, inaccurate way to say, "Over time, on islands, there was a reproductive advantage for smaller animals."
Why is it illogical to speak of evolution making something happen?
–Evolutionary change may be "blind" (unpredictable) but it is not random in a statistical sense. Consider how babies resemble their biological parents. The outcome is not predictable, but neither is it random.
The range of possible outcomes (when genetic expression is altered) depends on components and relationships that already exist. Those are not random influences.
Pre-existing components were built up, in some cases, over hundreds of millions of years. When people say evolutionary change is random what they really mean is that it is blind, not foretold, or surprising, just like the characteristics of a child are always somewhat novel and surprising, even if a child greatly resembles the parents.
How is evolution unpredictable, but not random?
–At the same time as the details of an evolutionary process are unpredictable, the general shape or outcome of many evolutionary process can be predicted. These predictions are the basis for annual flu vaccines. The flu virus mutates in a semi-predictable way, so each new vaccine is designed to fight the next anticipated variation.
When a particular evolutionary pattern is very advantageous, it may emerge independently several times. This is called evolutionary convergence. For example, the human gene for tolerating lactose in adulthood (so milk can be consumed by adults) has emerged at least twice in separate populations (Tishkoff et al., 2007), possibly four or more times (Wade, 2006).
What is convergent evolution? What does it mean to say two similar evolutionary outcomes are "analogous" vs. "homologous"?
Two structures or behaviors that appear the same, but evolved in different lineages independently, are said to be analogous. If, on the other hand, two parts are similar because of shared ancestry, that is called a homologous relationship. Because evolutionary convergence is always possible, DNA analysis is required to determine, for sure, whether similar-appearing traits are analogous or homologous.
People making an argument for an evolutionary explanation of human behavior commonly point to the ancestral environment for support. "This behavior would have been advantageous to our ancestors," goes the argument, and people will describe a primitive society where the behavior would have helped.
However, it is not so easy to specify the human ancestral environment, not because they did not exist, but because there were so many of them. Humans lived in many radically different environments over a short period of time (from the evolutionary perspective).
Evolutionary psychologists sometimes speak as if there was only one environment ancestral to them all: an EEA or environment of evolutionary adaptedness, a phrase suggested by John Bowlby. This is another case of evolutionary thinkers using a misleading shorthand reference.
It is important to think about ancient environments and their shaping influences, yes. But there was never just one ancestral environment.
What is an EEA?
Our pre-human ancestor Lucy, an australopithecus 3.2 million years ago, had a pelvic structure showing she could walk upright. She might have been comfortable strolling on a grassy savanna. However, her skeleton showed she could still climb easily in trees, with long strong forearms like a chimp.
Therefore, Lucy and her kind may have alternated between radically different environments, and they may have been comfortable in both. Their environment was also shaped by their own activities. Australopithecines like Lucy may have been able to use stone tools, as the earliest stone tools are found in strata between 2.6 and 3.3 million years old.
The earliest evidence of controlled fire is from the time of homo erectus, about 1.8 million years ago. This means all human ancestors for nearly two million years had an environment that included fire and stone tools. They hunted animals and gathered plants, and they had social groupings such as families, with all that implies.
Human and neanderthal lines diverged about 600,000 years ago. Rare interbreeding events afterward left us with 2% of more recent neanderthal genes in our DNA. The Denisovans were another pre-modern group that left genetic signs in the modern human genome. There are hints of another species, not yet identified, to add to the mix.
Modern humans are thought to have emerged from (and within) Africa at least 125,000 years ago. That date is continually pushed back. A generation ago modern humans were said to have emerged 60,000 years ago. Then it was 100,000 years ago. Now some experts say 200,000 years ago.
How long have modern humans been around?
Humans have low genetic diversity, compared to other primates. Chimps found a few miles apart in Africa have more genetic differences than humans from the opposite ends of the earth (Wilkins, 2012).
For years this low genetic diversity was thought to indicate a population bottleneck (near-extinction event) in the human past. Theorists suggested the population of modern humans might have bottomed out at 1-2,000 individuals, about 60-100,000 years ago.
That would account for the lack of human genetic diversity. Some suggested that humans were nearly wiped out by weather disruptions occurring worldwide after eruption of a huge volcano, Toba, in Indonesia 65,000 years ago.
However, a scientist who saw his research used to support the population bottleneck idea disagreed with it. John Hawks presented a statistical analysis of genetic changes (Hawks, 2008) indicating likely populations of 100,000 to 200,000 individuals during the entire period between 144,000 years ago and 70,000 years ago.
Hawks argued that populations started increasing around 144,000 years ago, but they may have been low (10,000 to 30,000 individuals) for a long time before that. Perhaps that accounts for our lack of genetic diversity, not a more recent bottleneck.
The earliest cave art, with hand prints and half-human, half-animal figures, is from about 36,000 years ago. Up to about 20,000 years ago, the ancestral social environment consisted of roaming bands of hunter-gatherers.
What was the human social environment from pre-human times until about 20,000 years ago?
Then came agriculture. A 23,000 year old site yielded clear evidence of agriculture, including "six shelters and a particularly rich assemblage of plants" including grains like rye. Six types of weeds were also found, and they are associated with cultivated gardens (Snir et al., 2015).
DNA evidence indicates dogs were domesticated from wild wolves at least twice (Grimm, 2016). Once was in Asia sometime over 14,000 years ago, another time was in Europe over 16,000 years ago. The two dog populations blended after that. Dogs greatly increased the efficiency of human hunters and also helped to protect fixed settlements.
Organized agriculture was spread widely by about 10,000 years ago. Adult lactose tolerance evolved in humans for the first time around 7,500 years ago, in Northern Europe. The EEA (ancestral environment) during that evolutionary event included herds of domesticated, milk-producing cattle.
As agriculture spread, human populations increased. Villages and cities appeared. People became attached to agricultural land and accumulated surpluses that had to be guarded with fortified architecture.
Trade routes were established to exchange surpluses. By 5000 years ago (3000 BCE) bronze tools had replaced stone tools almost everywhere.
Cultural change occurs much faster than biological change. When people started traveling great distances to exchange commodities like tin (needed for bronze), cultural change went with them.
In sum, our ancestral environment varied greatly over relatively short time periods (by evolutionary standards). Any time a writer speculates about behaviors that involved in a primitive enviroment, a legitimate question is, "Which environment are you talking about? Trees? Hunting? Traveling with migrating herds? Early farming life? Life in a Bronze Age city state? (etc)"
What question can be raised any time a person refers to our ancestral environment?
Probably the safest assumption is that patterns emerging prior to 20,000-
The result of all the variation in human ancestral environments may have been to encourage adaptability. Humans can live in any environment on earth, and they can change their culture as needed to survive.
Flexibility may be our greatest asset. It certainly distinguishes humans from other animals. No other species is found in such a wide range of environments, although penguins come close, being found in the antarctic and also on the equator (in the Galapagos Islands).
Grimm, D. (2016, June 2) Dogs may have been domesticated more than once. Science [Latest news blog] Retrieved from: http://www.
Hawks, J. (2008, May 2) Did humans face extinction 70,000 years ago? John Hawks Weblog Retrieved from: http://johnhawks.net/
Snir, A., Nadel, D., Groman-Yaroslavski, I., Melamed, Y., Sternberg, M., Bar-Yosef, O., Weiss, E. (2015, July 22) The origin of cultivation and proto-weeds, long before neolithic farming. PLOS ONE, 10. Retrieved from: http://journals.plos.org/
Tishkoff, S. A. et al. [19 authors] Convergent adaptation of human lactase persistence in Africa and Europe. Nature Genetics, 39, 31-40. doi:10.1038/ng1946
Wade, N. (2006, December 11) Lactose tolerance in east Africa points to recent evolution. New York Times. Retrieved from: http://www.nytimes.
Wilkins, A. (2012, March 4) io9.com Retrieved from: http://io9.gizmodo.
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Copyright © 2007-2017 Russ Dewey