The next 3 chapters (8,9, and 10) are further, very specific, examples of how common descent directly affects systems in our bodies. All mammals, reptiles, birds, and amphibians are descended from a common ancestor that had some traits. Now, all those thousands of species also share those same traits.
Sure, traits change over time and are adapted in different ways, from our manipulative upper limbs, to the wings of birds, and the flippers of aquatic mammals (and some extinct reptiles), the commonality is easily seen. It is difficult to image that the exact same structure is used in every single land dwelling (and former land dwelling) organism, without common descent.
In the past chapters we have seen how limbs, teeth, heads, even the common outline of our bodies (two forelimbs, two hind limbs, a head, a tail, a front and a back) all result from the earliest organisms. Even the fact that we have multicelled bodies can be seen by looking at the simplest one-celled organisms as they adapt to changing environments by combining together and the same proteins and genes that allowed them to combine together are used by our bodies to hold us together.
We have also seen how the concept of common descent is not based on one set of data or even one field of science, but on a body of work that encompasses hundreds of thousands of peer-reviewed research papers and fields as disparate as paleontology and molecular biology.
For now (Chapter 8), let’s consider smell. And we’re not talking about why fish tend to smell really bad after a few weeks in a freezer with no power (thanks hurricane Rita, I will always remember that).
I love the opening of this chapter, because it gives us a simple experiment that anyone can do at home and see DNA for perhaps the first time ever. I’ve done this in the lab, and while I haven’t done in the kitchen, the steps we used in the lab were exactly the same (except for some fancier chemicals than meat tenderizer and dish soap). You can actually see DNA (en masse) in your own kitchen. Get a kid and try it. The steps are easy.
- Take a steak or some peas or anything else for that matter add salt water and set your blender to “liquify”. This will disrupt the cells and make the insides of the cells available to us.
- Add some dish soap (non-lotion). This will break up the cell membranes that are too small for the blender.
- Add some meat tenderizer (un-flavored). This will break up the proteins that hold the DNA.
- Gently mix in some rubbing alcohol. The DNA is attracted to the alcohol (chemically speaking).
You should get two layers in your container. A layer of soapy water at the bottom and a layer of rubbing alcohol at the top. If you got a white ball in the rubbing alcohol, then you did it right. That white ball is pure DNA. If you are very careful, you can bend a paperclip, stick it into the ball of DNA and pull it out.
Congratulations, this is the basic step of genetic test and sequencing experiment ever done.
Why is this important to our sense of smell?
Humans have pretty good noses. The best among us can identify 10,000 unique odors. Some can pick out certain scents at 1 part in a trillion. That’s like picking out a single dollar bill out of one trillion. (And in this example, there are 100 sets of that (since those are $100 dollar bills)).
Scent works kind like a puzzle. Each chemical that we can smell has a unique shape and/or chemical attributes. We have proteins in the cells of our nose that matches those chemical perfectly. Then one of those chemicals latches onto the protein, the protein causes a signal to be sent to the brain. This signal tells our brain that the chemical has been detected and our mouth waters, or we vomit, or we remember grandma’s kitchen.
Now here’s the neat bit: 3% of our entire genome is dedicated to these receptor proteins. In other words, 3% of our entire genome is dedicated to the sense of smell.
Now each chemical has two different proteins that respond to it. One for the chemical in air and another for the chemical in water. Interestingly, fish have only water sensing proteins while land animals have only air sensing proteins and amphibians… well, they have both.
It gets even better. The researchers who found all those scent genes also discovered that, much like other genes and traits, they can be tracked via the mutations in them. Because they are so specific to chemicals, there is a limited amount of change that can occur in the gene before the ability to scent something is lost.
Humans have lost over 300 scents due to mutations in those scent genes. They are in the right place, the genes are just broken. Amazingly, creatures that would seem to be more closely related to humans have lost the same scent genes in the same ways (the same mutations).
Cetaceans present another interesting effect. It would make sense (from a design point of view) to change cetaceans (dolphins and whales) back to having water-based scent genes. But that isn’t the case. Of all the thousands of scent genes, not a single one is functional in dolphins and whales.
Evolutionary theory predicts that if you damage or mutate a gene that has no function, then the change will have no effect on the survivability of the organism. Unlike humans and (especially) other mammals, the sense of smell is nearly useless in cetaceans (especially sense all the genes are air-based). So losing them does not affect the fitness of the organisms.
To quote Shubin
inside our noses is a veritable tree of life.