In terms of macroevolution, comparative embryology can be very revealing. Take a look at how such divergent vertebrate species as fish, amphibians, reptiles, birds, and mammals look at very early embryonic states of development. In the early stages, all of them have gill slits, tails, and other similarities I won’t go into detail here. In fact, in the earliest stage shown below, you couldn’t tell a fish from a human if your life depended on it, could you? Yeah, the mammals seem a little thicker than the others but other than that, they are essentially identical. When comparing mammals against other mammals, it is virtually impossible to tell any differences. The examples shown below gave rise to the phrase: “Ontogeny recapitulates phylogeny” meaning that embryonic development reveals the more primitive state from which an organism evolved: fish, amphibian, reptile, bird, mammal, human, all of them being vertebrates, i.e., animals with backbones encompassing a spinal chord.
When you compare hemoglobins (blood protein) between different species, you see very similar relationships as well. Amino acids are biomolecules used to make proteins. They come together to form a chain which spontaneously folds on itself to make a functioning protein. As many as 20 such units can come together to form a single protein such as hemoglobin. The variety of such combinations verges on the infinite, similar to how we can form so many words using only 26 letters in our alphabet. The odds of two species having the same exact sequence of amino acids in one kind of protein is exceedingly small unless you take into consideration the theory of evolution. Note the differences in the amino acid sequences in hemoglobin from other animals compared to humans. Gorilla hemoglobin differs from our hemoglobin by only one amino acid. Just one! As you move away from the great apes into monkeys this variation increases by eight amino acids in monkeys. As you move away from primates into other mammals such as rodents the differences increase to 27. When you compare mammals to Aves (birds) the differences increase to 45. As we compare aminotes (land eggs or placental) to amphibians the differences increase to 67. When we compare differences to primitive jawless fish (lampreys) the differences are 1:125.
A similar phenomenon can be seen in DNA as well. Please click on the link below to see these comparisons.
Many more kinds of comparisons have been made between a variety of different kinds of bioomolecules. A thorough discussion of all of these comparisons would be beyond the scope of a simple blog such as this. Suffice it to say at this point that as more and more data is uncovered, the theory of evolution itself is beginning to evolve from just a mere theory into an actual law.