Asked in Evolution
What facts support evolution?
July 15, 2015 3:51PM
Wow, it's so hard to pick! The evidence for evolution is absolutely overwhelming, but I'll try and summarise some of the main facts:
- Classification: Organisms naturally fit into groups, e.g. humans naturally fit into the ape group, apes naturally fit into the primate group, then into the mammal group, the vertebrate group, the animal group, etc. This can easily be explained if we say that humans shared a recent common ancestor with all the other apes, then a more distant common ancestor with the other primates, then a more distant common ancestor with the other mammals, etc. It's not just humans; all organisms fit naturally into groups that show a kind of branching pattern, like a family tree. The most logical explanation for this is common ancestry - groups that seem more closely related shared a more recent common ancestor.
- Homologous structures: The human arm, the bird's wing, the whale's flipper and the cat's leg all share the same basic pattern of bones (one long bone, a joint, two small bones, a wrist made of lots of little bones and then five digits on the end) , even though they serve very different functions (human arms for grasping, bird wings for flying, whale flippers for swimming and cat legs for walking.) If they are built for very different tasks, why do they have the same basic structure? The answer is that humans, birds, whales and cats all share a common ancestor who had that kind of limb, and that over time, it's been modified to fit the needs of those four very different creatures. (There are many, many other structures too that have different functions but the same basic underlying structure, showing that they came from modifications from a common ancestor. These are known as homologous structures.)
- Genetic evidence: Perhaps the most powerful piece of evidence for evolution. Organisms that share more recent common ancestors, and are therefore more closely related, have more of their DNA in common. Humans, for example, share about 99.9% of our DNA with other humans, 98% with chimpanzees, 93% with rhesus monkeys, about 90% with mice, and so on... we share 50% with bananas! From this evidence we can draw a branching tree of who's more closely and less closely related to humans. Doing the same thing for other species allows us to work out who's related to who, and we get the same kind of tree we get from comparing similarities and differences (evidence 1) or looking at anatomical structures (evidence 2). Moreover, this is the same sort of technology used to convict criminals or in paternity tests - checking the similarities in DNA to find out who's related to whom.
- ERVs: ERVs, or endogenous retroviruses, are viruses that insert small bits of genetic code into the genome of the organism they infect. Biologists can easily see where an ERV has injected a bit of its own code, like a tag or mark. If ERVs infect a sperm or egg cell, then that genetic mark will be passed down to the children produced by that cell. By looking at the genetic markers left by ERVs in our genomes, we can therefore tell who shares a common ancestor with whom. And, of course, we see the same pattern - all humans share most of their ERV markers, some of them with chimpanzees, fewer of them with gorillas, etc.
- Embryology: In the first few days of development, a human embryo looks very similar to that of any other animal. After a few weeks, we can tell it's a vertebrate animal, but it doesn't yet look human - it could be a fish embryo! After a few more weeks, it looks like a mammal embryo, but we can't tell which mammal... and so on. Embryos of different species look very much alike in their early stages. The later on the embryos begin to look different, the more closely related the two species are. Further evidence comes when we look at some of the features embryos possess in their early stages. For example, at one point, human embryos have gill slits, like a fish (although never functioning gills), then later on a long tail, like most animals, then a yolk sac, like the eggs of birds and reptiles, and later still we become covered in fur like most mammals which we shed. Why would humans go through phases with gill slits, tails, yolk sacs and fur if none of these features are of any use to the adult? It's because our ancestors at one point had gills, tails, eggs with yolks, and fur (not at the same time, though!) We humans have kept those genes, and while they're inactive in adults, their effects show through at an early stage in development. And it's not just humans - embryonic snakes grow legs, which they lose, and horses grow five toes, of which four eventually shrink leaving behind the single hoof.
- Vestigial organs and behaviours: These are organs that an organism's ancestors used for one purpose but either serve a different purpose or are not used at all in the modern organism. A good example's the human tailbone or coccyx, which serves no function in humans but is what remains of our monkey's tail, or our appendix or wisdom teeth. There are vestigial behaviours, too: for example, if a human baby falls backwards it will automatically grasp something, perhaps a throwback to our days when falling out of trees was a concern. Or consider goosebumps, when our hair stands on end - not much good in warding off an enemy or trapping heat nowadays, but very useful for a furry mammal, which is why we get them when we're scared or cold. It's not just humans - whales for example have vestigial pelvises, although they have no legs for them to attach to.
- Atavisms: These are "evolutionary throwbacks," mutations that re-awaken old, deactivated genes and produce characteristics that an organism's ancestors had, e.g. humans with tails or fur.
- Pseudogenes: These are strands of DNA with no discernible function. Closely related organisms, like humans and chimps, or lions and tigers, share large amounts of this "junk DNA" with each other even though it doesn't seem to give them an advantage - and again, the pattern of who has the most junk DNA in common fits together well with the family trees (or phylogenetic trees) established from other evidence like anatomy, embryos, ERVs, etc. The only way to explain this again seems to be common ancestry.
- Molecular evidence: It's not just DNA that closely related organisms have in common; other molecules, such as cytochrome C, can vary from organism to organism and still be functional. More closely related organisms have the most similar cytochrome C. The same is true of blood proteins and other substances too.
- Human chromosome 2: Here's something that's specific to humans. The other great apes have a pair of chromosomes that humans don't. Where did it go? The only way evolution can explain it is if two of the great ape chromosomes fused together to make a human chromosome. When we look closely at the structure of the human chromosomes and those of the other great apes, we can pinpoint the exact place this happened - where two great ape chromosomes fused to make human chromosome 2.
- Ring species: For example, the Larus gulls; the Larus gulls in Europe can breed with those slightly different gulls in Asia, those in Asia can breed with those slightly different gulls further along in Asia, who can breed with those further along, and so on until we reach the east coast of North America. But those on the eastern coast of North America cannot interbreed with those on the west coast of Europe - they're too genetically different. This shows how it's possible for a species to change, very gradually, until it becomes a new species. Ring species like this show in space what must happen in time.
- Progression of the Fossil Record: When we look at the fossil record, we see organisms appearing in a sequence. For example, fish appear around 500 million years ago, amphibians around 360 million years ago, reptiles around 300 million years ago, early mammals around 200 million years ago, primates around 60 million years ago, apes around 30 million years ago, two-legged upright apes around 5 million years ago and humans around 200,000 years ago. Why no humans in rocks 500 million years old? The answer must be that humans evolved later. If humans evolved from apes, which evolved from primates, from mammals, from reptiles, from amphibians, and from fish, then the fish needed to appear first, followed by the amphibians, the reptiles, etc. The sequence of fossils in the rock record perfectly matches what we need to explain the tree of life.
- Transitional Fossils: Fossils that show a transition from one group to another group, e.g. Archaeopteryx, the reptile-like bird; Tiktaalik, the amphibian-like fish; Australopithecus, the human-like ape, etc. There are many, many, MANY more - just do a quick Google search!
- Age of the Earth: Darwin's theory of natural selection tells us that over time, and if the environment changes, it's inevitable that organisms will evolve and adapt to fit it. Countless pieces of evidence mainly from geology but also from physics, chemistry and astronomy show the Earth is ancient - 4.5 billion years old - and life is at least 3.5 billion years old. Given that natural selection and mutation must happen, it's absurd to think that organisms could have stayed the same for that length of time - evolution must have occurred on such an old planet.
- Evolution of bacteria: Many bacteria have evolved resistance to antibiotics through natural selection. Other bacteria have evolved the ability to digest nylon, which was only invented by humans in the 20th century. Why do we observe such changes happening so quickly in bacteria? Because they have short generation times - bacteria can produce a new generation in 20 minutes, while it takes more than 20 years for humans. This is why we have to wait for hundreds of thousands of years to see human evolution in action, but we can watch bacteria evolve over weeks or months in the lab or in nature.
- Peppered moths: A case study in natural selection. The white form of the peppered moth was the most common in England until that country industrialised. As tree trunks turned darker with soot and industrial pollution, white moths were more easily seen by predators, and so the darker moths were more likely to survive and reproduce. Over time, the population of moths grew darker over time. This is evidence that natural selection drives the change in the gene pool of populations that we call evolution.
- Geographical distribution of species: The way species are distributed tells us a lot about their evolution. For example, marsupials only exist in Australia and South America, although they could adapt to climates elsewhere. Why so restricted? It only makes sense if marsupials first evolved back when Australia and South America were connected to each other (and Antarctica, but not the rest of the world.) As the two continents broke apart, marsupials diversified on them, but never had the chance to spread anywhere else. (Marsupials in Antarctica would have died out as the continent froze over.) The same is true of many species; their geographical distribution only makes sense in evolutionary terms.
- Domesticated animals: These animals have been modified by humans over the years selecting who gets to breed and who doesn't. If humans can change an animal's gene pool through selection and produce such radically different types, why can't natural selection do the same thing?
- Sub-optimal design: Evolution cannot work from scratch, it can only modify/tweak pre-existing organisms' features for new needs. This is why so many parts of organisms' bodies aren't as efficient as they could be. For example, the blood vessels in the human eye, or the path taken by the human testes as they descend into the scrotum. If they were designed from scratch, there would be more efficient ways to do it; evolution had to make do with what was already there and tweak it to fit the new environments.
- Many, many more! Computer simulations of gene pools show evolution occuring; the structure of certain body parts can be fully explained by evolutionary processes, and any arguments against evolution (e.g. the "irreducible complexity" argument beloved by Intelligent Design advocates) have been shown again and again to not hold up to scrutiny. Since Darwin's time so many facts have been shown to support evolution and fit into an evolutionary scheme that as the great geneticist Theodosius Dobzhansky once said, "nothing in biology makes sense except in the light of evolution."