In terms of development mammals are weird. We have clearly evolved a set of wacky adaptations to allow us not only to mature entirely within the parent, but also to do so without the huge supply of yolk common to most other animals. Think of your regular egg. That big ball of yolk is covered by a single plasma membrane and provides everything that chicken needs for the roughly twenty-one days it takes to go from laying to hatching. As a freaky aside, that entire chicken (if fertilized) will grow from a small white mass (the blastodisc) visible on every single store-bought egg. Mammals, not having that whole bunch of rich and yummy stuff to eat in the weeks it usually takes them to develop, instead do a whole bunch of crazy things for which there is no real comparison elsewhere, except that a lot these structures (the amnion and alantois) are also present in the the eggs of birds and reptiles. What we’ve got here is a whole suite of adaptations used grow new organisms on land, which would be completely mind-boggling if you didn’t consider the 600 million years (that’s 219 billion days, to put it in perspective) that it has taken these systems to develop to their current state.
The means to accomplish these seemingly impossible tasks are aided by many hidden mechanisms, leading to my personal maxim that just because I can’t understand how something works doesn’t mean it defies explanation. Take heterochrony. From looking at the word you can probably tell that it has something to do with change and time. In developmental biology it’s the concept that a change in the timing of a developmental event can lead to evolutionary change. It accounts for a whole bunch of diversity, especially the oddball axolotl salamander of Mexico.
The idea has been explored by mucking about with metamorphosis in reptiles. If you take our above species with their external gills in their sexually mature form, subject them to the right chemical at the right time, they will actually develop into a ‘normal’ looking salamander without a pair of lungs sticking out the side of its head. Leave a couple of these treated salamanders to reproduce, and if you don’t interfere their offspring will look like their grandparents, face-lungs and all. Over an evolutionary timeline they’ve lost the ability to produce hormones that stimulate the thyroid. The experimental proof came from the treatment of axolotl with a precursor from a sheep, aptly named thyroid stimulating hormone. It’s also an elegant example of complex molecules that have distincct effects in various species. The hormone that influences your own metabolism is exactly the same in farm animals and even amphibians. If not the hormone thyroxine itself, then likely one of its precursors goes all the way back to the time when we first crawled out of the water.
So how does all this relate to the hatching of a mammal? The title is a double entendre. As mammals we hatch from the gelatinous zona pellucida as a blastocyst before implanting into our mother’s uterine wall. The more controversial meaning is in my intention to outline the principles by which humans could have evolved, and this is where my discussion of heterochrony is leading.
If humans evolved from the other primates in the past four to seven million years or so, how have drastic apparent changes come to pass in what you could say is a relatively short evolutionary timeline? We’ve lost most of our bodily hair and have a number of other retained juvenile traits. Click here for a scholarly reference if interested.
When I was younger I had assumed this involved the development of a suite of genes that specifically directed hair growth. The suite was already there, the evolutionary change was in turning it off at a certain time in certain cells. This does not require drastic genetic ‘rewiring’, but just a simple change not unlike what the axoltyl underwent; a loss of function mutation that resulted in a paedomorphic form; a mature juvenile form. In the salamander it’s more specifically called neoteny because they have a larval stage whereas we don’t undergo metamorphosis in our life-cycle. The advantage conferred by losing body hair is a discussion for another time, but next time someone questions our relationship with the apes, ask them what they think a hairless baby chimp might look like.
The mechanisms of evolution are coming to be understood better and better each day. Those who dismiss it as a ‘theory’ have an unfortunate misunderstanding of the nature of scientific language. Each decade a new mechanism is described that helps us understand the at first seemingly impossible task of generating advanced lifeforms capable of culture, language and altruism from the early chemical soup of the developing Earth. Heterochrony and the huge commonalities in the development of the many classifications of animals are only a couple of the many sources of empirical data that support the theory of evolution by natural selection. If you have a couple you would like to add, please be vocal about them, because apparently a great many people still need to be convinced.