When Charles Darwin set out on his voyage around the world in 1831, little did he know that his journey would be the catalyst for one of the greatest contribution to the field of science. Throughout the course of his expedition he encountered and collected many fascinating species of plants and animals, but it was what he found on the islands of the Galapagos Archipelago that changed the course of evolutionary biology forever. The islands were a melting pot of diversity, with species of plants and animals that were found nowhere else in the world.
Darwin soon noticed that though some plants and animals could be found on all the islands, each island held its own unique variation. Finches for instance, made their homes on every island, and on every island the finches were the same with the exception of their beaks. But why did the finches on the Galapagos Islands have such a huge variation in beak size and shape? And why were they so different from the finches found on the mainland of South America? On returning home to England after a long 5 year journey Darwin began formulating answers to these thought provoking.
After correspondence with experts in the field he would devise the Theory of Evolution by Natural Selection, which states that all organisms gradually change as time passes. As changes accumulate in a population new species are born from old ones. But his theory was in direct conflict with the beliefs of creationist, who believed that all living things were created at the beginning of the Earth’s history, and that they remained unchanged with the passage of time. So when he published his book “On the Origin of Species by Means of Natural Selection” in 1859, it was no surprise that creationists vehemently opposed his theory. Today Darwin’s theory is widely accepted within the scientific community, and that’s largely due to the overwhelming amount of evidence out there that supports it.
The Fossil Record
The fossil record is like an encyclopedia of life on Earth, albeit with missing pages. It gives us a glimpse at what came before us, millions of years ago. If evolution is indeed a process of nature then we should not only find evidence of ancestral species there, but we should also find a succession of them with slight variations. And that’s exactly what the whale lineage shows. The oldest known relative of whales preserved in the fossil record turns out to be a land dwelling mammal — distantly related to cows and hippopotamuses –called Pakicetus, which lived about 49 million years ago. It had legs just like any other terrestrial mammal, but its ears resembled those of modern whales. An adaptation that allowed Pakicetus to hear when submerged in water.
The second oldest fossil of whale ancestry shows a mammal that retained the same ear structure, but adopted expanded feet better suited to swimming. Ambulocetus nanans — as it was called — would have spent much of its time in the water, but would have ventured on land from time to time. Later, species like Maiacetus and Rodhocetus arose, with feet and spines that lent themselves to specialized swimming modes. Eventually though (around 40 million years ago) whales would abandon terrestrial life all together. As time passed random mutation and natural selection gave rise to Aetiocetus, which unlike its predecessors, had its nostril located in the middle of the skull rather than at the front. Like modern whales, whose blowholes are positioned at the top of the skull, Aetiocetus would not have had to completely surface in order to breathe. It’s a clear example of one species changing into another.
The Geographical Spread of Species
If the Theory of Evolution is true then the expectation is that species of plants and animals would only be found in places that their ancestors were able to travel to. And this prediction holds true. Though plants and animals such as monkeys and marsupials can be found on continents separated by vast stretches of ocean, despite the fact that they have no apparent means of the traversing these spaces, their presence there can be explained by taking a look at the Earth’s history. 275 million years ago the Earth’s landscape was entirely different. Instead of seven continents there was one supercontinent called Pangea, which was surrounded by a superocean called Panthalassa. But around 200 million years ago, the movement of the tectonic plates caused Pangea to break apart.
In its wake two smaller supercontinents were formed: Laurasia (Eurasia and North America) and Gondwana (Present day Africa, Australia, South America, Antartica and India). Eventually, these two super-continents slowly drifted apart until they became the seven continents we know today. Why is this important? Simply because prior to splitting apart the ancestors of groups of plants and animals, now separated by miles and miles of oceans, would have existed on these supercontinents and as the continents drifted apart so too did they. After a long period of separation natural selection acted on these separated populations and one species became two. This is why marsupials can be found as far apart as Australia and South America because they once shared the same space on the supercontinent Gondwana.
Evolution is not some magical process that can conjure new forms of life where there was none before. No, evolution can only act on pre-existing forms of life. If evolution is truly a process of nature and the lifeforms that are present today have descended from common ancestors, then their anatomies should show evidence of their shared history. And that is exactly what we see when we map the skeletal structure of vertebrates onto each other. Their skeletons — despite having different shapes and sizes — have the same types of bones, which are located in similar positions. How similar their structures are depend on how long ago those species diverged. Take the human hand for instance; it has five digits with each digit having four bones. If you look at the forearms of other mammals you’ll see a similar structure. A similarity that’s due to our descent from a common ancestor: Eusthenopteron.
Embryo Developmental Similarities
Despite having different forms as adults the embryos of dogs, snakes, fish, monkeys, and humans all look surprisingly similar during the early stages of their development. In the early stages of embryo development, the embryos of these animals possess structures that are eerily similar to gill slits and tails. But as the embryos progress into the later stages of their development, the gills slits are lost by all species with the exception of fish. On the other hand, great apes and humans both have tails during the early stages of embryo development, but those are lost some time before birth. These similarities are only possibly because they all share the ancient genes of a common ancestor. As these species diverged they accumulated different traits, but the blueprint for the beginning of life remained the same.
Discoveries in the field of genetics are probably the strongest evidence we have for evolution. It explains how mutations arise, how natural selection spreads them through a population, and more importantly how speciation occurs. When Darwin proposed the theory of evolution he still did not knew nothing about the mechanism that drove it. As time passed another would make the discovery of genes and their role in passing on traits to successive generations. And it would take another century or so for DNA to be discovered. In spite of there being thousands of diverse species, the code of life is shared universally. This strongly suggests that all life on Earth shares a common ancestor. The more similarities two species share in the sequence of their DNA the more closely related they are. That is why we humans share a whopping 96% of our DNA with chimpanzees, our closest relatives, but only 60% of our DNA with bananas.