If we look at the anatomy of various organisms, we can see evidence of clear developmental relationships.
For instance, all chordates possess the following: a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail .
In fish, the slits develop into gills. But for birds and mammals, they develop into the jaw and inner ear.
In humans and other apes, the post-anal tail is vestigial. But some other species make use of it for balance.
Why do all chordates have these? Why do they have all these features, even if for some species they are vestigial? And why have these features developed differently in later organisms and taken on different functions?
Because these are features which the common ancestor of all chordates possessed and thus all chordates continue to possess. Over time these features have evolved and some of them have taken on different roles as different groups evolved separately from each other. In some cases, these features became vestigial once they no longer served any purpose. Or as is often the case, these vestigial organs have been adapted for different purposes.
Yes, humans have tails! During embryonic development, this tail is highly visible. But during later embryonic development it is absorbed almost entirely without a trace. The only remnant of it is our tailbone.
Consider the fact that so many vertebrates have five digits. Including many creatures without a distinct hand or who have returned to the sea!
Why is this? Because their tetrapod ancestors had five digits on their forelimbs.
Take the fact that so many animals have four limbs! Why? Again, because of their ancestry.
If you look at humans, cats, bats and whales, they all have the following bones in the forelimbs: the humerus, radius, una, carpal bones, metacarpal bones and phalanges.
Why would a whale need all of these? Does a bat need four “finger bones”? They have these because their common ancestor did. Evolution does not typically completely do away with homologous features just because a given species does not have much need for them. Often such features remain as vestigial features.
The main point to appreciate here is that many animals have inherited basic anatomical features from common ancestors. Several of which modified as they evolved. But, unless they had common ancestors, that is unless evolution is true, how else do you explain such striking commonalities among seemingly disparate groups of organisms?
Then there are vestigial organs and skeletal structures. Such as the tiny splint bone in horses which serves no purposes but is a remnant of the time when horses had three toes. Or the appendix and tonsils of humans which do not serve their original purposes. As well as the tailbones of humans which serve no real purpose but is the remnant of the post-anal tail which all chordates possess.
Or the tiny bones in the hips of snakes and whales. These clearly do not serve their original purpose. Although they are often adapted to other purposes, like aiding in reproduction. They possess these because they had common ancestors with four limbs. Otherwise, why would they have these at all?
The geographic distribution of organisms on Earth exhibits patterns which provide evidence for evolution.
For instance, consider marsupials. They are found in South America, parts of North America, New Guinea and most notably Australia. Why here and not other parts of the world?
These were part of a land mass known as Gondwanaland, which split into several pieces 160 to 190 million years ago. The common ancestors of marsupials seem to have evolved there and had no path of migration out of Gondwanaland. As Gondwanaland split apart, populations of organisms were separated and took different evolutionary paths.
The Antarctic and South America both have fossil evidence that marsupials lived there in the past. Yes, Antarctica used to have marsupials. But these places are not where we find the most interesting marsupials.
Australia has a huge variety of unique marsupials. Why? Because the marsupial ancestors there were able to evolve in isolation from many other mammal groups and were able to diversify into an amazing variety of species without competition from non-marsupials. They fill many of the same niches filled by placental mammals in most other parts of the world. Whereas nowhere else on Earth seems to have developed such an amazing array of marsupial species.
This is evidence of evolution at work. How else would you explain the amazing array of marsupials found only in Australia? Except by the fact that they evolved as a result of their unique ecological isolation from other species.
Take the unique species found on islands such as the Galapagos and the Hawaiian Islands. These species have evolved in ecological isolation and not interbred with species on other islands. Thus, allowing them to evolve along divergent paths.
New Zealand has an amazing array of unique bird life not found in other parts of the world. From the tiny kiwi to huge species of moa larger than most ostriches!
How else would you explain this amazing diversity of species found in different parts of the world? Especially those found in landmasses and islands geographically isolated from others. Other than saying that evolution allowed for creatures to evolve in radically different paths when you start with different ancestors and or very different ecosystem
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