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The long march of evolution
The most profound study of evolution was made by an English naturalist, Charles Darwin (1809-1882), whose recognition of relationship through shared descent ranks as one of the great ideas in the history of science, now proven by modern genetics. Animals may look different from each other, but from fruit fly to humans, they use the same gene to establish their development pattern.
This is called the Hox gene because it contains a region of conserved nucleotide sequence known as homeobox. The conserved nature of the homeobox sequence shows the basic commonality of all life forms.
The evolution of the visual system is another good example of linkage between all life forms. The metazoans have generated eyes of diverse nature ranging from simple light-sensitive photoreceptors to compound eyes of insects. Despite this great variation, all of them are generated by a single master gene Pax-6, which was first identified in mouse.
Furthermore it has been shown that the induction of Pax-6 in ectopic sites of Drosophila (fruit fly) can generate eyes on various parts of its body. Experimentally it is possible to produce Drosophila with eyes on its legs, wings and antennae!
Darwin grew up in a family of biologists and his grandfather, Erasmus Darwin (1731-1802), was a well-known doctor and botanist. During his student days at Cambridge, Darwin spent a lot of time collecting beetles and plants. He was already an experienced naturalist when he went on his famous journey on HMS Beagle, which sailed around the world from 1832 to 1836.
While living aboard the Beagle he made copious notes of the fauna and flora that he observed. These- observations led him to formulate a theory of evolution, which he put in a book form. The book was entitled The origin of species and was published in 1859.
His theory centred around three postulates:
1. Individuals in every population group show variants;
2. Some of these variations are inherited by their offspring;
3. In each generation, more offspring are produced than can survive. The survivors survive because they possess favourable variants. This is the process of “natural selection”.
Darwin produced direct evidence of his theory by showing how the finches of Galapagos Islands differ in their beak’s shape and size under selection pressure, to obtain various types of food, ranging from seeds to insects. The English moth Biston betularia was another obvious example.
The dark coloured moths constituted less than 2 per cent of the population prior to the industrial revolution. Following the industrial revolution, darkening of the buildings occurred due to soot, and the population of dark moths grew to 95 per cent. The white moths were easily picked up by the birds against the dark background and were therefore almost all eliminated.
In modern times, one of the best examples of natural selection is seen in the common bacterium, Staphylococcus aureus, which inhabits our skin and sometimes produces serious disease. This bacterium was universally susceptible to Penicillin when this antibiotic was first introduced in the 40s.
Gradually, resistant strains of S. aureus started appearing and now, 60 years later, it is universally resistant. Because of the wide usage of Penicillin, natural selection eliminated all the susceptible strains. The end result is that S.aureus, as a species, survived because it had resistant variants in its original population, which multiplied under selection pressure. Evolution is, therefore, a preserver of some, and a destroyer of others.
Additional evidence in favour of change brought about by natural selection can be summarised as follows:
1. The fossil records of both animals and humans show distinct and progressive change, although complete phylogenetic trees are difficult to obtain for each species because of the time involved in the evolutionary process.
2. That changes have occurred is evidenced by the vestigial structures that exist in all animals. Humans have the tail bone (coccyx), a remnant of the ancestral tail, non-functional ear muscles, which were probably used for turning the ear towards sound, and a non-functional appendix.
The appendix is a very important part of the intestinal tract in herbivores, where the cellulose-splitting bacteria reside. As humans became omnivorous the need for a large appendix disappeared and the organ became vestigial. Erector pili muscles in human skin, which produce goose pimples when we get excited, are similar to the erectile fur of other primates which is used for demonstration of alarm or aggressive intent.
The goose pimples in humans no longer serve this purpose. In animals, there are many examples of vestigial organs. Some cave-dwelling fish have eye sockets but no eyes, and flightless birds have useless rudimentary wings. Snakes have tiny hip bones and rear legs in the form of claws, as they have evolved from reptiles. Recently, whale skeletons have been discovered in Pakistan with legs, indicating that whales were at some stage terrestrial animals. The University of Michigan palaeontologist who made this discovery has named the creature Pakicetus.
3. Selective breeding can bring about major changes in plants and animals. For example, a horse breeder can create faster horses by inbreeding the fastest amongst his stock. This happens because organisms differ from each other and the selected variations are inherited. In nature too, selective breeding occurs, which Darwin called natural selection, eliminating the weak and favouring the “fittest” — the fittest generally being those who succeed in mating and passing their genes to their offspring.
There is unanimity of opinion among biologists that natural selection does occur, but can it alone account for the production of all the characteristics that we see in an array of animal and plant species? Let’s turn to Darwin’s predecessor, Jean-Baptiste Lamarck (1744-1829), for possible answers.
Lamarck was a firm supporter of evolution and in his praise Charles Darwin wrote in 1861: “... He (Lamarck) first did the eminent service of arousing attention to the probability of all changes in the organic, as well as in the inorganic world, being the result of law, and not of miraculous interpretation.”
Lamarck is best known for his publication, Theory of inheritance of acquired characteristics, which was published in 1801. He proposed that the changes brought about by the environment are passed on to the offspring, and it is the environment which brings about evolutionary changes.
His theory centres around the principles of use and disuse. In our lifetime, if one regularly exercises, certain muscles develop. A blacksmith’s arms are usually more well developed than his legs. Similarly, a runner’s leg muscles are more developed than his arm muscles.
Are these characteristics inherited? Are the offspring of a runner born with hypertrophied leg muscles? The answer is obviously no, but a Lamarckian could say that for such a change to occur it would take many thousands of years. There is no way to disprove this.
The fact that children are born with a foreskin although circumcision has been practised for centuries does not disprove Lamarck because there is a difference between external damage and an internal stimulus to change. There is an aspect of Lamarcism, which is quite intriguing.
He spoke of animals striving towards a change or a “tendency to perfection”. The giraffe eventually developed a long neck because it was trying to eat the leaves from tall trees. In a sense, the animal was “needing” to evolve in a particular direction, and there is therefore a mental component to the evolutionary change.
One could say that if this was true, why hasn’t the Eskimo developed fur like the polar bear? He hasn’t developed fur, but certainly has a thicker layer of subcutaneous fat, as compared to the African, and has narrow eyes and double eye lids, to protect him from the glare of the snow. The Eskimo’s needs have been fulfilled!
This could, however, have happened due to natural selection, as many evolutionary biologists would argue. According to Lamarck, the “true principles” to be noted in his theory are:
1. Alteration in the environment causes alteration in the needs of animals;
2. Change in needs causes new activities to develop to satisfy those needs;
3. Every new need necessitates either more frequent use of some parts which develop and enlarge or the development of entirely new parts, to which the needs have imperceptibly given birth by effort of its “inner feeling”. In short, the sequence is environment-need-new activities-physical change.
4. The opposite is also true. “The permanent disuse of an organ, arising from a change of habits, causes a gradual shrinkage and ultimately the disappearance and even extinction of that organ.”
It is interesting that Aristotle, (384-322 BC) had suggested that there is a design in nature, but it is less from guidance from without, than the urges from within. It is the inner drive or “entelechy” which leads to the fulfillment or the fullest realization of a being.
There is a great deal of scientific evidence that the central nervous system has a profound effect on the immune system and the working of other organs of the body. For instance, stress in animals can release cytokine (IL-I) from the macrophage, and these are the earliest molecules involved in the inflammatory response.
The examination stress in students causes a significant drop in the number of natural killer (NK) and T-helper cells. Similarly, depression has an adverse effect on immunity. Mind/body researcher Lydia Temoshok studied the psychological factors associated with malignant melanoma.
She found a correlation between emotions and the growth of cancer cells. We now realize that a vast number of diseases are psychosomatic in origin (psyche is mind, soma is body). In other words, thoughts and emotions do get translated into physical phenomenon.
Andrew Schwartz, a neurophysiologist at Arizona State University has recently reported that monkeys implanted with special electrodes in their motor cortex can move a cursor on a computer screen just by thinking about it. They were previously trained to use their hands for this purpose, but after their hands were strapped, they could still do it by using their mind.
If the mind and body are so intimately connected, why can’t the mind also affect the gene, which is a physical component of the cell? This possibility cannot be totally dismissed. Darwin and Lamarck can both be correct.
Another giant in the field of evolution was Ernest Haeckel (1834-1919). Haeckel was trained as a physician but abandoned his practice in 1859 after reading Darwin’s book and concentrated on the study of evolution.
Haeckel agreed with Darwin’s idea of evolution, but was not very enthusiastic about natural selection. Instead, he believed that the environment had a crucial role in the formation of races, a view closer to Lamarck. He expounded the idea that “ontogeny recapitulates phylogeny”. That is, embryonic development is a replay of that animal’s evolutionary past.
We now know that this is not strictly correct, and new features unrelated to ancestry may appear or disappear during embryogenesis. Nevertheless, there are structures, which clearly reveal evolutionary connections with the past. For example, all vertebrates start as a single cell, then become multicellular (morula stage) and then get curved with a protruding tail (tail-bud stage).
In the tailbud stage, all embryos have gills like that of the fish embryo. Later, the gills change into proper gills in fish, and gill bars in humans and other animals. The gill bars form the various structures of the face, mouth and ears. The embryos also develop fin-like structures, which turn into proper fins in the fish, and limb buds in other animals.
After birth, the human babies show features similar to those of other primates. Both are born with the grasp reflex, which disappears in humans, but persists in other primates. The reflex in the human baby is elicited when the baby’s palm is stroked, to which it responds by making a fist.
The reaction is so strong that the baby can be lifted by one’s finger, if it is slipped into his or her palm. The grasp reflex in human babies disappears in about two months; during the course of evolution, it has lost its significance. But in other primates it is a life saving reflex.
Baby monkey would fall to its death unless it holds on to its mother’s coat while she is jumping from tree to tree. Therefore, the grasp reflex in the monkey persists, until the baby is strong enough to manage on its own.
According to the anatomist Tryphena Humphrey, of the University of Pittsburgh, the first cutaneous reflex of the foot appears in the 10.5-11 week-old-foetus in the form of planter flexion of the toes. This reflex largely disappears in later foetal life, but is sometimes seen in newborn infants when it is referred to as a foot grasping reflex. This “undoubtedly represents an equivalent of the foot-grasp reflex found phylogenetically in birds and some arboreal mammals”.
Human babies and chimpanzee babies are remarkably similar in their physiognomy. A newborn chimpanzee has big doleful eyes and a delicate face, but it later develops a protuberant face and massive jaws. The meeting point, as far as facial features are concerned, occurs in the first few months after birth, subsequent to which the species move in different directions.
Adult monkeys are quadruped; apes are partially biped; and humans are completely biped. But the human infant starts as a quadruped (crawls) and then gradually becomes a biped, again repeating its evolutionary past. Even in adulthood, humans involuntarily swing their arms while walking, in unison with their legs, as their quadrepedal memory has not totally disappeared.
An interesting evidence for Haekel’s idea of evolution came recently from observations made by Gaeth, Short and Renfree of the department of zoology and perinatal medicine, University of Melbourne, Australia, and reported in the Proceedings of the National Academy of Sciences in 1999. It is generally believed that elephants were originally aquatic animals, and then they became terrestrial.
In Melbourne University they studied the histology of elephant embryos from animals culled at the Kruger National Park of South Africa. The embryos ranged from 0.04-18.5 grams, with estimated gestational ages from 58-166 days (full gestation in elephants is 660 days). The study revealed that the embryonic kidney of the elephants has a structure known as nephrostome, which is a tiny funnel-shaped duct, characteristic of aquatic animals.
This structure was extensively present in the embryonic kidney for a few weeks, a sign that elephants had an aquatic past, which was being repeated in their terrestrial present. The authors also observed that the trunk appeared at the same time as nephrostome. It is therefore conceivable that the trunk evolved as an adaptation to the aquatic environment and was used as a snorkel when they swim in deep water even today.
Irrespective of the theories that exist about evolution, it is almost certain that no animal or plant appeared on Earth in its present form. The long march of evolution, which started at the time when life began on this planet, goes on unabated. This is a fundamental law of nature.
The article has been published with permission from World Scientific, the publishers of Prof Zaman’s book Life sciences for the non-scientist
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