Any individual protein doesn't seem that complex since it's just a combination of some 20 amino acids, but the variations are endless:
"Since each of the 20 amino acids is chemically distinct and each can, in principle, occur at any position in a protein chain, there are 20 × 20 × 20 × 20 = 160,000 different possible polypeptide chains four amino acids long, or 20n different possible polypeptide chains n amino acids long. For a typical protein length of about 300 amino acids, more than 10^390 (20^300) different polypeptide chains could theoretically be made. This is such an enormous number that to produce just one molecule of each kind would require many more atoms than exist in the universe."
proteins are also unique in that not just their sequence matters, but also their physical shape. 2 proteins can have the same sequence but a different physical shape, and therefore have different impacts on the body's chemistry. I started a PhD researching DSP methods for matching protein sequences and locations of amino acids. Fun stuff.
Then there are also post translational modifications, like addition of acetyl or phosphate groups, and sugars to the protein (glycoproteins).
I mean, I can understand how an eye or a brain can evolve by natural selection, but I’m still stunned by abiogenesis. I guess we’ll never know for sure how it all started.
"Since each of the 20 amino acids is chemically distinct and each can, in principle, occur at any position in a protein chain, there are 20 × 20 × 20 × 20 = 160,000 different possible polypeptide chains four amino acids long, or 20n different possible polypeptide chains n amino acids long. For a typical protein length of about 300 amino acids, more than 10^390 (20^300) different polypeptide chains could theoretically be made. This is such an enormous number that to produce just one molecule of each kind would require many more atoms than exist in the universe."