Humans are undeniably complex, and proud of it. No case, we believe, needs to be made for our biological superiority. Our biological functions are exquisitely regulated and resilient to external variations, owing to complicated webs of interactions. Unlike other species, we seem to be endowed with willpower and intellect, hence we are capable of modifying the environment to buffer the effects of our decreasing fitness.
Be that as it may, we may be doomed as a species precisely because of the way in which our complexity arose. Paraphrasing the science writer Philip Ball, nature seems to have activated a time bomb, and our complexity is only a short-term fix.
To grasp the nature of the problem, we need to examine how humans are made at the molecular level, and contrast our constitution with that of other species that we often call “rudimentary,” such as unicellular organisms. This analysis leads us to examine proteins – our cellular building blocks and the executors of biological functions – across vastly different species. Proteins with common ancestry belonging to different species, termed “orthologs,” offer solid ground for comparison.
It has been generally recognized that the basic “fold,” or shape, of a protein must be conserved across species, because there is a tight correspondence between structure and function. Proteins that retain the same function across very different species – generally the case with orthologs – are expected to keep the same fold.
But the sequence of amino acids that make up the protein chains in these orthologs can vary significantly. Sometimes the extent of sequence identity between two orthologs can be as low as 25-30%, and yet their folds remain strikingly similar, attesting to the robustness of function to evolutionary change.