The Real Barrier to Unguided Human Evolution
Comparing DNA sequences and estimating by how many nucleotides we differ from chimps doesn't tell us much about what makes us human. Many of those nucleotide differences have no effect, because they are the product of neutral mutation and genetic drift. While these neutral mutations may affect the over-all mutation count, they don't answer how many mutations are required for the transition from chimp-like to human.
This problem is analogous to one we examined concerning protein evolution last year in the journal BIO-Complexity (Gauger and Axe 2011). Converting one protein to another's function can be viewed as a version, in miniature, of converting one species to another. But it is much easier to convert proteins than species.
We began by identifying two proteins that are close together in structure, but that have distinct functions. We examined what the minimal number of mutations to convert one protein to the other were. If all the places where they differed had to be changed, that would mean we would have to switch 70% of one protein to achieve conversion to the other's function. It's unlikely that all those mutations are required, however, since many if not most of those changes are due to neutral mutation and drift, just like in the chimp-like to human case.
So to estimate the minimal number of mutations required for conversion to a new function, we identified and tested the most likely amino acid candidates using structural and sequence comparisons, one by one and in combination. We ended up changing nearly the entire active site to look like the target protein, but failed to achieve conversion.
Based on the number of groups we changed, we made a minimum estimate that seven specific mutations would be required for a functional shift to be observed. To get seven coordinated mutations takes far too long, even for bacteria, with their high mutation rate and large population sizes. 10^27 years is our estimate, based on Doug Axe's population genetics model, also published in BIO-Complexity.
Personally, I think the chimp-like to human conversion would have to have taken many more years than any protein conversion, if it happened at all. A few years back Durrett and Schmidt published two papers in which they estimated how long it would take to get a single mutation, and then a second mutation to produce an eight base DNA binding site somewhere within a thousand base region near a gene. They stipulated that within the thousand base region there already was a sequence with six out of eight bases matching the target. The reason they chose to examine DNA binding sites? Many evolutionists think that evolution happens by changing gene expression, and changing gene expression most often requires changes to the regulatory regions around genes.
Their results? They calculated it would take six million years for a single base change to match the target and spread throughout the population, and 216 million years to get both base changes necessary to complete the eight base binding site. Note that the entire time span for our evolution from the last common ancestor with chimps is estimated to be about six million years. Time enough for one mutation to occur and be fixed, by their account.
To be sure, they did say that since there are some 20,000 genes that could be evolving simultaneously, the problem is not impossible. But they overlooked this point. Mutations occur at random and most of the time independently, but their effects are not independent. Mutations that benefit one trait may inhibit another. In addition, many if not all these traits are complex adaptations. Each trait requires multiple mutations to achieve a beneficial change. And many of the traits must occur together to be of any benefit. Take, for example, the changes required for upright bipedalism. Hips, legs, feet, spine, ribcage, skull all need to work together to allow free and efficient motion. All must be changed. But changing the hips before changing the angle of the legs would not be helpful. Changing to upright posture without lengthening the neck and setting the skull atop the spine would not work.
The point is this. There are hundreds of traits that distinguish us from chimps, probably requiring tens of thousands of mutations in total. But even if it takes only 30 or 40 specific trait changes to move from primate to human, and hundred of mutations, the time required would be astronomical. Longer than the age of the universe, actually.
Like Richard Sternberg's argument about whales, the argument from what is required to what is possible shows there just isn't time enough for it to have happened by unguided means.
Image credit: Vitruvian Man, Leonardo da Vinci/Wikicommons.
Cross-posted at Biologic Perspectives.