Compare a horse to a human. Compare any mammal for that matter and you'll see we share a lot in common.
Comparative Anatomy — The Strongest Evidence of Common Decent
Take the arm of a human and compare that to the wing of a bat, the flipper of a porpoise, or Fido's front paw, they all have the same bone structures arranged in a similar way, even though we all use them differently. All species of mammals have a humerus bone, the upper part of the arm, radius and ulna below the elbow, wrist carpals, hand metacarpals and finger bones.
A bird, bat, a butterfly comparatively all have wings and are made to carry out the same job but are completely alien to each other, with different origins.
In our early embryonic stages, a halibut egg lying on the ocean floor, a fresh laid chicken egg on Farmer Dell's farm, a human fetus, we all have a common ancestry, we all had tails and gills. In humans, those tails and gills disappear by the eighth week using a process called "Programmed Cell Death."
Luckily, today there's Genetics exposing DNA as the strongest evidence of a common decent. A field of study that didn't exist during Darwin's lifetime. All living things have DNA and RNA, hereditary material. It's no more a secret, we all come from a common ancestor, there's a biochemical similarity in every living thing's basic processes.
On the evolutionary time line, anatomical mutations in a species may or may not occur. Birds evolved from reptiles about 150 million years after mammals, so you'd never find a mutant mammal with feathers but then you might find a whale, since whales have evolved from mammals, with legs. There's a possibility the whale might have limbs instead of flippers, which on rare occasions, happen!
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