Evolutionary Theory in 21st CenturyPresentation Transcript
This year marks the 150th anniversary of the publication of Origin of species as also the bicentenary of Charles's Darwin's birth. A lot has happened in the last 150 years. Evolution is accepted as a fact (like apples falling to the ground) by near 100% of all Biologists. Evolutionary Theory in 21 st Century
Even the Catholic and Anglican churches, once the greatest foes of Darwinism, now accept Evolution as much more than a hypothesis. The fact that they no longer regard the Biblical account of creation as literal truth is testimony to the immense advances that Biology has made in the past century and half. In fact many people feel that Biology has replaced physics to become the leading science of the 21 st century.
Biology today is capable of performing miracles. This jellyfish has a protein called Green Fluorescent Protein (GFP) that enables it to glow bright green in the dark. Gene for this protein can now be cut and pasted to the gremlin of a mouse. See what happens.
Half the babies born to the mouse in the previous picture have inherited the chromosome containing the newly introduced GFP. They glow bright green in the dark.
Remember small pox? During the 20th century, it is estimated that smallpox was responsible for 300–500 million deaths.
A disease with no effective treatment, people prayed to Godesses like Sheetala Mata and Mariyamman to no avail.
It was with the advent of effective vaccination by Edward Jenner and a major campaign by the World health Organization that the disease was removed from the face of the earth. A great triumph for the Biological sciences!
Christopher Reeve famous for his role as Superman became confined to the wheelchair after an accident that injured his spinal cord. He became an ardent advocate of stem cell research which promises new hope for those paralyzed by spinal cord injuries. Today his dream is close to becoming reality. Scientists can take embryonic stem cells and make them differentiate into nerve cells. This can be transplanted to the injured spinal cord to replace lost cells.
Life expectancy was below 40 years in the mid nineteenth century in Europe and America. It has increased to nearly 80 years in a century and half. This is spurred entirely by the changes brought about by our knowledge of biology, in the form of medical and public health interventions.
The story of this great advance in biology has its origin in the famous voyage of this ship – the HMS Beagle and the young naturalist aboard on that voyage – Charles Darwin .
The ship in its voyage lasting 5 years circumnavigated the globe, spending most time in South America.
The most famous stop in the journey was in the Galapagos islands off the coast of Ecuador.
In the Galapagos Darwin found birds like Finches which were closely related to those in the South American continent, but different species. There was diversity between each island. The most important differences between species are in the size and shape of their beaks, which were highly adapted to different food sources abundant in each island. Darwin realized this only later after his return to England. The Finches along with the Galapagos tortoise helped him construct the theory of natural selection.
The giant tortoise is the best known of all Galapagos animals and even gave the archipelago its name; 'Galapago' means tortoise in Spanish. Galapagos giant tortoises can weigh up to 250 kg and live for more than 100 years. They are thought to belong to just one species, with 14 different races or sub-species.
There are a great variety of carapace shapes, from very large domes to smaller saddle-shaped shells, with all gradations in between. The different shapes probably evolved as an adaptation to the particular environment of each island.
These tortoises illustrate the principal of natural selection that Darwin theorized after insights in the Galapagos. Populations isolated on islands or on parts of larger islands have adapted to different conditions and now have distinct appearances. The species can be generally separated into those with 'domed' shells, which occur on the larger, wetter islands with abundant vegetation on the ground, and smaller tortoises with 'saddleback' shells that are found on smaller islands with dry vegetation. It is thought that the distinctive saddleback shell enables its bearer to reach taller vegetation; these tortoises also have longer limbs and necks.
Think of an island in which there were 1000 tortoises to begin with. Both variations were present. The dome shaped with short necks much more numerous, say 95%; the longer necked being rare. Now if changes in the ocean causes repeated flooding and destroys ground level vegetation, the long necked individuals have better survival chances and hence better reproductive success. If the conditions persist, within few generations the long necked variation becomes more numerous and becomes the dominant type in the island.
Alfred Russel Wallace, a British naturalist and explorer during his travels in South East Asia reached very similar conclusions as Darwin. Their findings were read together at the meeting of the Linnaean society in London on 1 July 1958. In the picture is Wallace and his drawing of the Bird of paradise.
Darwin’s ‘The Origin of species by means of Natural selection’ was published on 24 th November 1959. It is a detailed argument for evolution of organisms the course of generations through a process of natural selection. It presented a body of evidence that the diversity of life arose through a branching pattern of evolution and common descent through a vast period of time. It became one of the most influential books ever written.
Fossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The hard parts like bone, teeth and exoskeleton of insects fossilize relatively easily, given the right conditions. They get progressively mineralized and often turn into rock. Because the process is extremely slow, the shape and even finer details are preserved. Shown here is the fossil of a trilobite which belonged to a group of extinct arthropods.
Fossils were known much before Darwin's time. They were once believed to represent remains of dragons and other mythical creatures. Aristotle and Leonardo Da Vinci considered them to be remains of animals who once inhabited the earth. It was soon realised that in the earth's past there lived strange animals no longer seen today. The famous French scientist, Georges Cuvier maintained that fossils proved the existence of worlds previous to ours each destroyed by a series of catastrophes.
Salagrama (Shaligram) Sila – black or red stones – found in the Gandaki river in Nepal, a tributary of Ganga.
It is worshipped by Hindus as a special manifestation of the God Vishnu
Salagrams are fossils of marine animals that belonged to a group known as Ammonites. They appear in rocks of about 400 million years ago. They are no longer seen in rocks younger than 65 million years ago. This means that they became extinct at the same time as the dinosaurs. But how is it that the remains of these marine animals seen in a river bed atop the Himalayas – so far fro any ocean? The answer to this illustrative of how scientific evidence from different fields add up to give intellectually satisfying answers.
We know now from studies in geology and paleontology that the continents are not fixed, but move continuously by the mechanism now known as plate tectonics. About 45 million years ago, the Indian Plate collided with Asia, buckling the crust and forming the Himalayas. This accounts for the remnants of marine animals in the Himalayas.
These are the old world monkeys like the typical Rhesus monkey seen in the Eastern hemisphere. They have three color vision like the apes and humans.
The New World monkeys seen in the Eastern hemisphere are different in many ways. They do not have three color vision. They have prehensile tails with which they can grip tree branches. It is believed that the ancestors spread from the Old world when the Atlantic ocean was much narrower. They got isolated from their old world cousins as the Atlantic ocean widened. Thereafter each group evolved in their separate ways.
This is the family tree of the monkeys and apes. Platirhini or the New world monkeys split from the African branch about 40 million years ago. The Other branch devloped into old world monkeys and apes. Such family trees can be constructed fairly accurately using studies of changes in DNA sequences.
Probably no creatures that have lived upon this Earth have excited the imagination more than the Dinosaurs. For some 150 million years they dominated the land, evolving into a wide range of forms and populating every continent. We are fascinated by them because they often had such bizarre forms, because they sometimes grew to such huge size, perhaps because after ruling the Earth for so long they suddenly vanished about 65 million years ago. We have abundant fossil evidence regarding the evolution of Dinosaurs. Shown here is the skeletons of a huge Sauropod Dinosaur and the fearsome tyrannosaurus rex.
And a reconstruction of how they would have looked like in real life.
Some nineteenth century clergymen claimed that the Dinosaur bones were planted by the Devil to trick the believers. But now even the fossilized soft parts of some dinosaurs have been unearthed. This is the skin of a hadrosaur, or duck-billed dinosaur, discovered in 1999 in North Dakota in the United States.
It is not merely the dinosaurs. This is the skeletal remains of an animal that has become extinct only been recently – the woolly mammoth.
Many carcasses of the woolly mammoths have been found buried under the Siberian ice. This is the best preserved of them all – a 6 month old female baby mammoth discovered in 2007. It had lived some 40000 years ago. Because of the well preserved DNA, scientists are thinking whether they can be cloned in the wombs of Indian elephants to whom they are closely related, like in Jurassic park, thus bringing live an extinct animal for the first time.
The woolly mammoths co-existed with human beings. Pre-historic cave paintings depict them. We were partly responsible for their extinction.
This is the Grand Canyon, a steep gorge formed over millions of years by the Colorado River cutting into the rock. It is in Arizona, United States. Here you can see exposed rock representing about 2 billion years of earth’s history. The older rocks are at the bottom and the recent ones at the top. Gradual changes in the evolution of a group can be seen in the strata. The age of these rocks and fossils can be independently dated using various radiometric methods.
Here you can see a series of fossil horses, the ones below in the older rocks and the ones above in the newer. The evolution from Eohippus (Hyracotherium) that lived about 60 million years ago to the modern horse (Equus) is shown. There is progressive change from four toes to one toe on the front feet. Changes in size and shape of the skull and body, along with teeth morphology, also happened in parallel. The gradation in a series of progressively younger strata is strong evidence for evolution. If the fossils are in a random order, say four digits followed by one and then two etc. it would have been proof against descent with modification (evolution). Fossil evidence for gradual change is known for many living species and in no case is the scheme of geochronology violated.
One of the favorite arguments of anti-evolutionists is to point out the lack of transitional fossils. For example till recently they used to ask “How can a land animal like a cow or buffalo or pig turn into a whale? Can you even imagine a transitional form?” Well you don’t have to imagine. Recent discoveries like Pakicetus and Ambulocetus in Pakistan now gives us a fairly good idea of the steps in evolution of whales from land animals.
Fossils that bridge the gap between major groups are also well known. Archaeopteryx has features of birds (feathers) and Dinosaurs (tail, teeth) and can be considered an early bird that evolved from the dinosaurs.
Tiktaalik is a lobe finned fish that forms a link to the four legged land animals. See that the bones in the fin are similar in distribution to that of the limbs of land animals. The reconstruction of the extinct fish shown below.
Intelligent design Intelligent design is an advocacy movement that says “The universe and living things are best explained by an intelligent cause, not an undirected process such as natural selection.“ The idea was propagated by a group of American creationists who reformulated their argument in the creation-evolution controversy to circumvent court rulings that prohibit the teaching of creationism as science.
Examples abound in nature that imperfections of design exist as natural selection is never perfect enough to take care of all exigencies. Recurrent laryngeal nerve that runs from the brain to the larynx, helping us speak doesn't take a direct route but descends into the chest, loops around the aorta near the heart, and then runs back up to the larynx. In the giraffe the nerve has to traverse the neck twice and so is fifteen feet long-fourteen feet longer than necessary! The added length makes the nerve more susceptible to injury, and its tortuous path makes sense only in light of evolution. We inherited our developmental pathway from that of ancestral fish, in which the precursor of the recurrent laryngeal nerve attached to the sixth of the gill arches. During mammalian evolution the blood vessel in the sixth gill arch moved backward into the chest, becoming the aorta. As it did so, the nerve that looped around it was constrained to move backward.
It has been pointed out that the human eye is not of perfect design. The nerves pass in front of the retina with its light receiving photoreceptors and where they join together to go backward to form the optic nerve, there is a blind spot. A human engineer would perhaps have designed the eye in a different way. This is another example of how natural selection is ‘give and take’ sometimes sacrificing certain things to attain certain ends.
This is the giant Squid - a fearsome predator of the seas - which has the largest eyes of all animals. They have eyes that are superficially similar to our eye, but on close examination, we find that all the details are different. In particular, they have the "wiring" on the back of their retina, as you'd expect. There is no blind spot, and the entire retina is fully functional.
The evolution of the eye has been a problem since the days of Darwin. He openly worried about the lack of intermediate forms. Later creationists have pounced on his idea and have asked ‘What use is half an eye or quarter of an eye to an animal. Now many intermediates are known. In different mollusks (the family of squid and octopus) different forms starting from a simple layer of photoreceptor epithelium to the perfect squid eye can be seen. Even the simple layer of photoreceptors are useful for the organism to sense source and direction of light.
Ostrich wings are vestigial organs no longer used for flight, though it may have other uses like keeping balance when running. The human appendix is more of a nuisance than of use, being the cause of appendicitis that can cause death if not treated. People who have their appendices removed have no health problems or any decrease in life span.
Human appendix is the vestige (remnant) of a much larger chamber in herbivorous animals, used for bacterial digestion of cellulose. As we lost the ability to digest cellulose, the appendix was no longer needed and became smaller as natural selection was no longer bothered about its presence.
The revolution in molecular biology now provides overwhelming evidence for common descent and progressive change of all life forms.
All these life forms are so different, giving the appearance that they were all created separately. And yet they all share features that can best be explained from common descent.
All differences between organisms are due the differences in various proteins in each of them. Proteins are composed of amino acids arranged one after the other. The order in which amino acids are arranged determines the shape of that protein and thus its function and properties.
All proteins in living beings on this planet is composed of the same 20 amino acids. Amino acids can be left handed (L) or right handed (D) in its property to rotate polarized light clockwise or anticlockwise. Both forms can theoretically function in life. But almost all amino acids found living beings are the Left handed form. This can only be explained by common descent from a single ancestor.
The order of amino acids in proteins in decided by the sequence of nucleotides in the DNA of the genes.
There are only 4 nucleotides. Adenine (A), Thymine (T), Guanine (G) and Cytosine (C) in the DNA. In RNA Uracil (U) takes the place of Thymine. It is ultimately the variation in their sequence that produces all variation in living beings.
One of the greatest discoveries of science is the deciphering of the genetic code. Three nucleotides in any particular sequence stands for an amino acid. Thus 64 possibilities are there, most of the coding for one of the 20 amino acids. UUU stands for Phenyl Alanine, UUA for Leucine etc.
Monkey or ameba, the tree or ant or us the genetic code is the same. How else to explain this startling fact other than surmise that we have all inherited the protein synthetic machinery that arose in the first cell. The fact that we are very much part of nature and share kinship with all other living beings can be a very uplifting and beautiful thought. It also bestows on us responsibility for the preservation of life in all its myriad forms.
Hemoglobin is the protein in the red cells of blood that takes oxygen to our tissues. It is a protein composed of 4 chains – 2 identical alpha and 2 identical beta chains. The beta chain has 146 amino acids. Let us see the extent of difference between us and other animals.
0/146 1/146 2/146 8/146 No difference whatsoever with the chimpanzee. 1 amino acid is different in the Gorilla. 2 in the gibbon and 8 in the rhesus monkey.
25/146 15/146 27/146 45/146 67/146 125/146 Amino acids are different in 15 position in the dog compared to us. 25 in the horse. 27 in the mouse, 45 in the fowl, 67 in the frog and 125 in the lamprey. This is almost exactly as the evolutionary trees built up by comparative anatomy, paleontology etc. predict.
24x2 24x2 24x2 24x2 These are our closest living cousins; the apes - Chimpanzee, Gorilla, Orang-Utan and Gibbon. All of them have 24 pairs of chromosomes. We on the other hand have 23 pairs. 23x2
If we line up all the chromosomes of apes and humans we see that 22 pairs are almost exactly the same. We have a large chromosome two which the apes don’t have. Instead, they have two smaller chromosomes which have been named 2A and 2B.
Chromosome banding is a technique which produces alternating dark and light bands, the patterns of which are unique to each chromosome. This is a technique for identification of chromosomes. If 2A and 2B of the apes are aligned in a particular way, they are nearly identical to the human chromosome 2. The only way to explain this is by assuming that in an ancestor of humans after divergence from the apes, the chromosomes 2A and 2B fused together to form a single larger chromosome.
Testable prediction: Common ancestor had 48 chromosomes (24 pairs) and humans carry a fused chromosome; or ancestor had 23 pairs, and apes carry a split chromosome. Centromere Telomere Ancestral Chromosomes This hypothesis can now actually be tested. When two chromosmes fuse they would have two centromeres. But only one can be functional and the other gets inactivated. But centromeres have unique nucleotide sequences. If our argument is true, the human chromosome should contain and inactivated centromeric sequence. Fusion Homo sapiens Inactivated centromere Telomere sequences
“ Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two acrocentric chromosomes that remained separate in other primates. The precise fusion site has been located in 2q13–2q14.1 (ref. 2; hg 16:114455823 – 114455838), where our analysis confirmed the presence of multiple subtelomeric duplications to chromosomes 1, 5, 8, 9, 10, 12, 19, 21 and 22 (Fig. 3; Supplementary Fig. 3a, region A). During the formation of human chromosome 2, one of the two centromeres became inactivated (2q21, which corresponds to the centromere from chimp chromosome 13) and the centromeric structure quickly deterioriated (42).” Homo sapiens Inactivated centromere Telomere sequences Hillier et al (2005) “Generation and Annotation of the DNA sequences of human chromosomes 2 and 4,” Nature 434: 724 – 731. Human Chromosome #2 shows the exact point at which this fusion took place Indeed this was found as brought out in this paper published in 2005.
We can now reconstruct the histroy of human – ape lineage. The Hemoglobin beta chain mutation leading to the difference in one amino acid between Human and Gorilla occurred in a common ancestor of humans and chimps after the line leading to Gorillas separated. The chromosome fusion event occurred in a human ancestor after the chimp lineage branched off.
New Fossils and molecular biology are shedding further light on our ancestry. The data from the Human genome project and the successful sequencing many other genomes including that of the chimpanzee and mouse bound to enrich our knowledge further. We will look at human evolution and what it means to be human in the next lecture.