ADVERTISEMENTS:
Darwin formulated the principles of natural selection and evolution, which revolutionized biology. Müller’s insistence that biology must be an experimental science provided the starting point for an important tradition. However, other biologists continued to observe, classify, and think about what they saw, and some of them arrived at valuable conclusions. The most important of these scientists was Charles Darwin.
Functionalism and the Inheritance of Traits:
Darwin’s theory emphasized that all of an organism’s characteristics—its structure, its coloration, its behaviour— have functional significance. For example, their strong talons and sharp beaks permit eagles to catch and eat prey.
Most caterpillars that eat green leaves are themselves green, and their colour makes it difficult for birds to see them against their usual background. Mother mice construct nests, which keep their offspring warm and out of harm’s way. Obviously, the behaviour itself is not inherited—how can it be? What is inherited is a brain that causes the behaviour to occur.
ADVERTISEMENTS:
Thus, Darwin’s theory gave rise to functionalism, a belief that characteristics of living organisms perform useful functions. So, to understand the physiological basis of various behaviours, we must first discover what these behaviours accomplish.
To understand the workings of a complex piece of machinery, we should know what its functions are. This principle is just as true for a living organism as it is for a mechanical device. However, an important difference exists between machines and organisms — Machines have inventors who had a purpose when they designed them, whereas organisms are the result of a long series of accidents. Thus, strictly speaking, we cannot say that any physiological mechanisms of living organisms have a purpose, but they do have functions, and these we can try to determine.
A good example of the functional analysis of an adaptive trait was demonstrated in an experiment by Blest. Certain species of moths and butterflies have spots on their wings that resemble eyes—particularly the eyes of predators such as owls. These insects normally rely on camouflage for protection; the backs of their wings, when folded, are coloured like the bark of a tree.
However, when a bird approaches, the insect’s wings flip open, and the hidden eyespots are suddenly displayed. The bird then tends to fly away, rather than eat the insect. Blest performed an experiment to see whether the eyespots on a moth’s or butterfly’s wings really disturbed birds that saw them. He placed mealworms on different backgrounds and counted how many worms the birds ate. Indeed, when the worms were placed on a background that contained eyespots, the birds tended to avoid them.
ADVERTISEMENTS:
Darwin formulated his theory of evolution to explain the means by which species acquired their adaptive characteristics. The cornerstone of this theory is the principle of natural selection. Darwin noted that members of a species were not all identical and that some of the differences they exhibited were inherited by their offspring.
If an individual’s characteristics permit it to reproduce more successfully, some of the individual’s offspring will inherit the favourable characteristics and will themselves produce more offspring. As a result, the characteristics will become more prevalent in that species. He observed that animal breeders were able to develop strains that possessed particular traits by mating together only animals that possessed the desired traits.
If artificial selection, controlled by animal breeders, could produce so many varieties of dogs, cats, and livestock, perhaps natural selection could be responsible for the development of species. Of course, it was the natural environment, not the hand of the animal breeder that shaped the process of evolution.
Darwin and his fellow scientists knew nothing about the mechanism by which the principle of natural selection works. In fact, the principles of molecular genetics were not discovered until the middle of the twentieth century. Briefly, here is how the process works – Every sexually reproducing multicellular organism consists of a large number of cells, each of which contains chromosomes.
Chromosomes are large, complex molecules that contain the recipes for producing the proteins that cells need to grow and to perform their functions. In essence, the chromosomes contain the blueprints for the construction of a particular member of a particular species. If the plans are altered, a different organism is produced.
The plans do get altered; mutations occur from time to time. Mutations that affect the development of offspring are accidental changes in the chromosomes of sperms or eggs that join together and develop into new organisms. For example, cosmic radiation might strike a chromosome in a cell of an animal’s testis or ovary, thus producing a mutation that affects that animal’s offspring.
Most mutations are deleterious; the offspring either fails to survive or survives with some sort of defect. However, a small percentage of mutations are beneficial and confer a selective advantage to the organism that possesses them. That is, the animal is more likely than other members of its species to live long enough to reproduce and hence to pass on its chromosomes to its own offspring.
Many different kinds of traits can confer a selective advantage-resistance to a particular disease, the ability to digest new kinds of food, more effective weapons for defense or for procurement of prey, and even a more attractive appearance to members of the other sex.
Naturally, the traits that can be altered by mutations are physical ones; chromosomes make proteins, which affect the structure and chemistry of cells. But the effects of these physical alterations can be seen in an animal’s behaviour. Thus, the process of natural selection can act on behaviour indirectly.
ADVERTISEMENTS:
For example, if a particular mutation results in changes in the brain that cause a small animal to stop moving and freeze when it perceives a novel stimulus, that animal is more likely to escape undetected when a predator passes nearby. This tendency makes the animal more likely to survive and produce offspring, thus passing on its genes to future generations.
Other mutations are not immediately favourable, but because they do not put their possessors at a disadvantage, they are inherited by at least some members of the species. As a result of thousands of such mutations, the members of a particular species possess a variety of genes and are all at least somewhat different from one another. Variety is a definite advantage for a species.
Different environments provide optimal habitats for different kinds of organisms. When the environment changes, species must adapt or run the risk of becoming extinct. If some members of the species possess assortments of genes that provide characteristics that permit them to adapt to the new environment, their offspring will survive, and the species will continue.
Evolution of the Human Species:
To evolve means to develop gradually. The process of evolution is a gradual change in the structure and physiology of plant and animal species as a result of natural selection. New species evolve when organisms develop novel characteristics that can take advantage of unexploited opportunities in the environment.
The first vertebrates to emerge from the sea—some 360 million years ago— were amphibians. In fact, amphibians have not entirely left the sea; they still lay their eggs in water, and the larvae that hatch from them have gills and only later transform into adults with air-breathing lungs.
Seventy million years later, the first reptiles appeared. Reptiles had a considerable advantage over amphibians- Their eggs, enclosed in a shell just porous enough to permit the developing embryo to breathe, and could be laid on land. Thus, reptiles could inhabit regions away from bodies of water, and they could bury their eggs where predators would be less likely to find them.
Reptiles soon divided into three lines- the anapsids, the ancestors of today’s turtles; the diapsids, the ancestors of dinosaurs, birds, lizards, crocodiles, and snakes; and the synapsids, the ancestors of today’s mammals. One group of synapsids, the therapsids, became the dominant land animal during the Permian period.
Then, about 248 million years ago, the end of the Permian period was marked by a mass extinction. Dust from a catastrophic series of volcanic eruptions in present-day Siberia darkened the sky, cooled the earth, and wiped out approximately 95 percent of all animal species. Among those that survived was a small therapsid known as a cynodont—the direct ancestor of the mammal, which first appeared about 220 million years ago.
Mammals (and the other warm-blooded animals, birds) were only a modest success for many millions of years. Dinosaurs ruled, and mammals had to remain small and inconspicuous to avoid the large variety of agile and voracious predators. Then, around 65 million years ago, another mass extinction occurred. An enormous meteorite struck the Yucatan peninsula of present-day Mexico, producing a cloud of dust that destroyed many species, including the dinosaurs.
Small, nocturnal mammals survived the cold and dark because they were equipped with insulating fur and a mechanism for maintaining their body temperature. The void left by the extinction of so many large herbivores and carnivores provided the opportunity for mammals to expand into new ecological niches, and expand they did.
The climate of the early Cenozoic period, which followed the mass extinction at the end of the Cretaceous period, was much warmer than it is today. Tropical forests covered much of the land areas, and in these forests our most direct ancestors, the primates, evolved. The first primates, like the first mammals, were small and preyed on insects and small cold-blooded vertebrates such as lizards and frogs.
They had grasping hands that permitted them to climb about in small branches of the forest. Over time, larger species developed, with larger, forward-facing eyes, which facilitated arboreal locomotion and the capture of prey. As fruit-bearing plants evolved, primates began to exploit this energy-rich source of food, and the evolution of colour vision enabled them to easily distinguish ripe and unripe fruit.
The first hominids (human-like apes) appeared in Africa. They appeared not in dense tropical forests, but in drier woodlands and in the savanna—vast areas of grasslands studded with clumps of trees and populated by large herbivorous animals and the carnivores that preyed on them. Our fruit-eating ancestors continued to eat fruit, of course, but they evolved characteristics that enabled them to gather roots and tubers as well, to hunt and kill game, and to defend themselves against other predators.
They made tools that could be used to hunt, produce clothing, and construct dwellings; they discovered the many uses of fire; they domesticated dogs, which greatly increased their ability to hunt and helped warn of attacks by predators; and they developed the ability to communicate symbolically, by means of spoken words.
The first hominid to leave Africa did so around 1.7 million years ago. This species, Homo erectus, scattered across Europe and Asia. One branch of Homo erectus appears to be the ancestor of Homo neanderthalis, which inhabited Western Europe between 120,000 and 30,000 years ago. Neanderthals resembled modern humans. They made tools out of stone and wood and discovered the use of fire.
They encountered the Neanderthals in Europe around 40,000 years ago and coexisted with them for approximately 10,000 years. Eventually, the Neanderthals disappeared — perhaps through interbreeding with Homo sapiens, perhaps through competition for resources. Scientists have not found evidence for warlike conflict between the two species.
Evolution of Large Brains:
Our early humans’ ancestors possessed several characteristics that enabled them to compete with other species. Their agile hands enabled them to make and use tools. Their excellent colour vision helped them to spot ripe fruit, game animals, and dangerous predators.
Their mastery of fire enabled them to cook food, provide warmth, and frighten nocturnal predators. Their upright posture and bipedalism made it possible for them to walk long distances efficiently, with their eyes far enough from the ground to see long distances across the plains.
Bipedalism also permitted them to carry tools and food with them, which meant that they could bring fruit, roots, and pieces of meat back to their tribe. Their linguistic abilities enabled them to combine the collective knowledge of all the members of the tribe, to make plans, to pass information on to subsequent generations, and to form complex civilizations that established their status as the dominant species. All of these characteristics required a larger brain.
A large brain requires a large skull, and an upright posture limits the size of a woman’s birth canal. A newborn baby’s head is about as large as it can be. As it is, the birth of a baby is much more arduous than the birth of mammals with proportionally smaller heads, including those of our closest primate relatives. Because a baby’s brain is not large or complex enough to perform the physical and intellectual abilities of an adult, it must continue to grow after the baby is born.
In fact, all mammals require parental care for a period of time while the nervous system develops. The fact that young mammals are guaranteed to be exposed to the adults who care for them means that a period of apprenticeship is possible. Consequently, the evolutionary process did not have to produce a brain that consisted solely of specialized circuits of nerve cells that performed specialized tasks.
Instead, it could simply produce a larger brain with an abundance of neural circuits that could be modified by experience. Adults would nourish and protect their offspring and provide them with the skills they would need as adults. Some specialized circuits were necessary, of course, but by and large, the brain is a general-purpose, programmable computer.
What types of genetic changes are required to produce a larger brain? As we know that, the prenatal period of cell division in the brain is prolonged in humans, which results in a brain weighing an average of 350 g and containing approximately 100 billion neurons. After birth, the brain continues to grow.
Production of new neurons almost ceases, but those that are already present grow and establish connections with each other, and other types of brain cells, which protect and support neurons, begin to proliferate. Not until late adolescence does the human brain reaches its adult sizes of approximately 1400 g—about four times the weight of a newborn’s brain. This prolongation of maturation is known as neoteny. The mature human head and brain retain some infantile characteristics, including their disproportionate size relative to the rest of the body.