Jose Fly
Fisker of men
I wrote this stuff years ago, so I'm just going to throw it in this thread for anyone who's interested....
Regarding the nature of the atavism itself...
How evolutionary theory explains the genetic basis for the atavistic tail.....
Regarding the nature of the atavism itself...
Atavisms are a concept similar to vestigial structures. But atavisms take it a few steps further. Atavisms are the reappearance of a characteristic not present in the parents or even in recent evolutionary relatives. IOW, let's say you are an amphibian that evolved from a species of fish that existed millions of years ago. An atavistic structure would be if a recent amphibian individual were born with pelvic fins like those of its piscine ancestor.
Similarly, if humans share an evolutionary past with other primates, the types of atavistic structures we have should reflect this evolutionary past. More specifically, our evolutionary past allows us to predict the types of atavistic structures in humans we should find and shouldn't find. We shouldn't see a baby born with fully functional wings for example.
So, is this what we see?
Of course. The most famous example of atavisms in humans is the appearance of fully functional tails (also known as "coccygeal process," "coccygeal projection," "caudal appendage," and "vestigial tail"). The existence of these true tails are extremely well documented in the medical literature, with over 100 published examples.
Keep in mind, these are REAL tails, complete with adipose and connective tissue, central bundles of striated muscle, blood vessels, nerve fibres, nerve ganglion cells, and specialized pressure sensing nerve organs. They are covered by normal skin, complete with hair follicles, sweat glands, and sebaceous glands. And guess what else? These things actually work! They contract and move just like regular tails we see in other primates!!
Similarly, if humans share an evolutionary past with other primates, the types of atavistic structures we have should reflect this evolutionary past. More specifically, our evolutionary past allows us to predict the types of atavistic structures in humans we should find and shouldn't find. We shouldn't see a baby born with fully functional wings for example.
So, is this what we see?
Of course. The most famous example of atavisms in humans is the appearance of fully functional tails (also known as "coccygeal process," "coccygeal projection," "caudal appendage," and "vestigial tail"). The existence of these true tails are extremely well documented in the medical literature, with over 100 published examples.
Keep in mind, these are REAL tails, complete with adipose and connective tissue, central bundles of striated muscle, blood vessels, nerve fibres, nerve ganglion cells, and specialized pressure sensing nerve organs. They are covered by normal skin, complete with hair follicles, sweat glands, and sebaceous glands. And guess what else? These things actually work! They contract and move just like regular tails we see in other primates!!
How evolutionary theory explains the genetic basis for the atavistic tail.....
If humans share a common ancestor with other primates, we would expect to see this reflected in our ontological development. The development of a human adult and an ape adult from an embryo are modifications of the developmental processes of our shared common ancestor. In other words, the common ancestor that humans share with other primates went through a certain process in its embryological development. Because we and other primates have descended from this ancestor, all primates (ourselves included) should have inherited components of this process. Thus a basic prediction of our shared ancestry is that our ontological development should be very similar to that of other primates.
So, is this what we see?
A great example is how all primates have a tail at 4-5 weeks of gestation. At this stage there are 10-12 developing tail vertebrae that extend beyond the anus and legs, and are greater than 10% of the length of the whole embryo. The tail is made up of many complex tissues including a spinal cord, a notochord, a mesenchyme, and a tail gut. In the monkeys, this goes on to form the various types of tails present in adult monkeys.
But as we know, the great apes (including humans) do not have tails. In the great apes, the 6-12 vertebrae undergo "cell death" and disappear, and the 5th and 4th vertebrae are reduced. It has been shown that regulation of a single gene (Wnt-3a gene if you need to know) is the mechanism behind this curious process (thus the cases of the atavistic tails I mentioned earlier are likely due to a mutation in this gene). I say "curious process" because this data leads to the obvious question: Why develop a tail as an embryo that you're not going to have as an adult?
This is a very good example of how evolutionary common descent of all primates provides a very good explanation for the data. Without an understanding of common descent, the ontological development and loss of a tail as an embryo in all apes would be very strange and difficult to explain. But when we view this data in light of common ancestry, it makes perfect sense. At one time, all primates had tails as embryos that went on to develop into adult tails. But sometime after the great ape line split off, a common ancestor to all apes developed a mutation that resulted in the embryonic degeneration of the tail, and that mutation and trait was inherited by all its descendants, humans included.
So, is this what we see?
A great example is how all primates have a tail at 4-5 weeks of gestation. At this stage there are 10-12 developing tail vertebrae that extend beyond the anus and legs, and are greater than 10% of the length of the whole embryo. The tail is made up of many complex tissues including a spinal cord, a notochord, a mesenchyme, and a tail gut. In the monkeys, this goes on to form the various types of tails present in adult monkeys.
But as we know, the great apes (including humans) do not have tails. In the great apes, the 6-12 vertebrae undergo "cell death" and disappear, and the 5th and 4th vertebrae are reduced. It has been shown that regulation of a single gene (Wnt-3a gene if you need to know) is the mechanism behind this curious process (thus the cases of the atavistic tails I mentioned earlier are likely due to a mutation in this gene). I say "curious process" because this data leads to the obvious question: Why develop a tail as an embryo that you're not going to have as an adult?
This is a very good example of how evolutionary common descent of all primates provides a very good explanation for the data. Without an understanding of common descent, the ontological development and loss of a tail as an embryo in all apes would be very strange and difficult to explain. But when we view this data in light of common ancestry, it makes perfect sense. At one time, all primates had tails as embryos that went on to develop into adult tails. But sometime after the great ape line split off, a common ancestor to all apes developed a mutation that resulted in the embryonic degeneration of the tail, and that mutation and trait was inherited by all its descendants, humans included.
