KF81 said:
I thought i would post it as he raises some good questions that i do not know the answer too !
"There are visible mutations present in bacteria and viruses all the way up to insects. We know it happens, but it takes many generations to accomplish. The biggest problem with humans is that we haven't had significant time to mutate and specialize as quickly as the geological record indicates.
The claim in this quote, that there hasn't been enough time for humans to evolve, is problematic. Our knowledge of the details of our own evolution, and of evolution in general, is not complete enough to support this claim. We know a great deal about the general mechanisms that enable evolution to occur, but we remain uncertain about how much time it takes for different traits to evolve and spread in complex organisms.
Not only that, but when we talk about structures like the brain, we have to start taking into account
social pressures on evolution. That is, evolution of the brain can be influenced by the structure of society and the ways that people interact with each other and society as a whole.
KF81 said:
Modern humans very quickly appeared and dominated all other hominids. How do we explain the advent of the prefrontal cortex? What natural stressor created the need for such a mutation in only the highest order of mammals? What was the main driver of that mutation, and how did it happen so quickly?"
I don't know what timescale the person has in mind. Human evolution stretches back about 5-10 million years ago (mya), the time our ancestors split off from chimpanzees. There is evidence of bipedalism from around 4 mya, perhaps as early as 7 mya, and brain size began to noticeably increase around 2.1 mya as seen in fossils of Homo Habilis. Homo Erectus fossils (1.9 mya) continue the trend by having even larger brains than Homo Habilis, and the general pattern is one of increasing brain size as we get closer to modern day.
To try to shed some light on the questions from the quote I'm going to go at them one at a time:
1.
How do we explain the advent of the prefontal cortex?
First, it's important to examine
how we might go about answering this sort of question. The first thing we can do is to look at the function of the modern prefrontal cortex (PFC). A very basic rundown is that the PFC is involved in highly complex and abstract tasks such as complex planning (like your ability to plan out your day ahead of time), personality expression, decision making, and moderating social behavior. To quote a paper on the prefrontal cortex (link below):
"The prefrontal cortex receives highly processed information from all major forebrain systems, and neurophysiological studies suggest that it synthesizes this into representations of learned task contingencies, concepts and task rules. In short, the prefrontal cortex seems to underlie our internal representations of the 'rules of the game'."
Next we might examine a.) the physical differences in the PFC between species and b.) the differences in the above mentioned abilities between species.
The PFC is
much larger in humans than in other primates (and by extension other animals), even after adjusting for absolute brain size. This size difference also correlates with how much complexity an animal exhibits in the behaviors and abilities linked to the PFC, with larger PFC's correlating with more complex behavior. I'd give an example of the differences, but you
really need to read the paper I've linked below. The authors of the paper do a far better job of explaining this than I can.
Source:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1693009/
Knowing that differences exist in both behavior and physical structure, we can make an educated guess that the prefrontal cortex evolved to facilitate more complex behaviors. That is, mutations that resulted in larger and more complex brains and PFC's endowed the organism with the ability to handle more complex tasks, better regulate its own behavior, understand the environment a little better, etc, all of which are beneficial. This then leads to an increased survivability of the organism and its offspring, helping to spread the mutation throughout the population. Obviously such a guess would need to be studied in-depth to determine whether it is viable or not.
2.
What natural stressor created the need for such a mutation in only the highest order of mammals?
This question demonstrates a misconception that the person has regarding evolution. Evolution works by building off of what is already available. Large changes involving hundreds or thousands of new proteins and large-scale restructuring of DNA, culminating in substantially different body designs and functions, simply do not happen within the time span of even a few million years. You simply won't find them, as the chance that such a huge number of beneficial mutations occur without also incurring at least an equal number of detrimental, even
lethal, mutations is vanishingly small. You might as well say that the first complex cell arose by sheer chance from a pool of simple molecules. (that didn't happen either)
The reason I mention this is because the kind of changes in the PFC seen in the human lineage were only able to occur
because the PFC was already as complex as it was. That is, the existing complexity of the PFC in the primate brain directly enabled the subsequent mutations to generate an even more complex PFC. And this isn't limited to the PFC. The entire brain structure of our primate ancestors had to simultaneously evolve along with the PFC, with the changes building off of what already existed at the time. Less complex organisms can't simply evolve a larger, more complex brain without also going through a very long process involving not only the brain, but the rest of the body, potential social structure, and many other factors. To put it bluntly, OUR LINEAGE is this exact process in action. One could say that the process of evolving a species with as complex a brain as ours from an amphibian species takes roughly as long, and involves roughly the same number and scale of changes, as it took for
us to evolve from amphibians.
Finally, we have to remember that evolution is still dependent on mutations that occur randomly within the genome. It's entirely possible that the chimpanzee brain is perfectly capable of evolving into one just as complex as ours, but that
the right mutations, in the right order, and subject to the right pressures, simply haven't occurred for them.
3.
What was the main driver of that mutation, and how did it happen so quickly?
The answer to that question is simply not known at this time. It's very difficult to distinguish between cause and effect when looking back hundreds of thousands or millions of years at species that we only know of through a small number of fossilized remains. And we haven't been studying evolution for long enough to observe such large changes in a species. But that doesn't mean we can't try to answer the question, and scientists are doing exactly that as we speak. A number of references on this exact topic are given in the encephalization section of the wikipedia page on human evolution:
https://en.wikipedia.org/wiki/Human_evolution#Encephalization
The questions posed in the quote are indeed good questions and scientists are currently working to answer them. However, the fact that questions still exist is not necessarily a 'problem'. That is, it does not detract from evolution in a general sense and, as yet, does not detract from the details of human evolution as posed in various models. Science's entire purpose is to answer all the questions that arise when you start to look into how the universe works. There will
always be unanswered questions as long as we have the capability of doing meaningful science.
KF81 said:
"There are visible mutations present in bacteria and viruses all the way up to insects.
I just noticed this as I was finishing up the rest of the post above. I don't know why the person stopped at insects when referring to mutations. Every single species that we can observe, including ourselves, show numerous mutations. Just about every cell in your body has a slightly different DNA makeup thanks to mutations that occur during the cell's lifetime and upon cellular reproduction. Even cancer is a product of evolution in the sense that a population of cancerous cells evolved from a population of non-cancerous cells through the mutation of several key genes. There's even an
infectious (i.e. can be transmitted) type of
cancer that affects dogs that evolved from plain old non-cancerous canine cells. One could potentially classify this as a different species, consisting of a unicellular, asexually reproducing pathogen.
