These are short chapters and not quite as intense as some of the others, so I’ll combine them into one post. Plus, these two chapters are intimately related as I will describe, in some detail. After this will be Chapter 11, which ties everything together.
So far, we have seen 8 chapters worth of evidence for common ancestry and here we get 2 more.
Chapter 9 – Vision
Much like the evolutionary history of our noses, the basics of our vision system are present in bacteria.
Keep in mind that all our eyes do is gather light and focus the light onto the retina. The retina is full of special cells called rods and cones. These rods and cones are specialized light absorbing cells, each responding to certain forms of light and sending nerve impulses to the brain. The brain then interprets the pattern of nerve signals into what we think of as an image.
As far as the eye itself, we can trace the evolutionary history from simple light gathering cells on the surface of a planaria to the highly intricate (and better than ours) eyes of the octopus.
But that’s not the really cool bit. The molecules that actually absorb the light energy and covert that to a electrical impulse which can start a nerve transmission are called opsins.
Opsins are found in almost all organisms on the planet. Humans, insects, clams, and scallops all use opsins (yes, scallops have eyes).
We can even go a bit farther than that. Evolutionary principles can be used to make predictions. We would predict that organisms that are more closely related to humans would have more similar vision systems. Indeed, we find this to be the case. Humans have three genes for opsin production, giving us a very large range of color vision.
This is indeed the case, with the majority of mammals have two genes for color vision and the higher primates (yes, including humans) having three.*
There is a much deeper common relationship among eyes in the animal kingdom. There are huge differences between vertebrate ‘camera-style’ eyes and invertebrate compound eyes. Until 2001, this might have been considered an unbridgeable gap and many prominent biologists assumed that eyes developed twice in the history of multi-celled life on the Earth.
But in 2001, Detlev Arendt found that a polychaete worm had BOTH kinds of vision systems… well the basics for each. The worm has a ‘normal’ invertebrate eye with connections to the neurons and opsins but, underneath the skin, were tiny photoreceptors with vertebrate opsins and tiny bristle like projections that matches basic rods and cones. This animal has the precursors to both vision systems in use by the animal kingdom.
Finally, we turn to genetics (as we always seem to). Research into fruit flies produced the discovery of a mutated gene that causes a reduction or the elimination of eyes in fruit flies. The eyeless gene was very interesting. By turning ON this gene in cells in various places in the fruit fly, research could grown an eye there. Whole generations of fruit flies were born with eyes on their legs and abdomen and mouths and wings. Some of these eyes, could even respond to light and considering they aren’t actually connected to the nervous system, that’s pretty cool.
Another gene was discovered in mice, Pax6. It’s the mouse equivalent of eyeless. With some creative genetic engineering, they found that Pax6 could be used to grow an eye in fruit flies (again, anywhere the researchers wanted), but it was still a fruit fly eye.
These two genes, one in a fly and one in a mouse, were so similar that either of them can trigger the complex developmental cascade that results in the formation of an eye.
Chapter 10 – Ears
This is chapter 9 and we find the same kind of thing here. We can trace the developmental history of the structures of our ears all the way back to the gill arches in fish. There are three bones in our middle ear, the malleus, incus, and stapes (or hammer, anvil and stirrup if you remember that better).
I’d like to point out that what we’re talking about right now was discovered by Karl Reichert in 1837, twenty-two years before Darwin published On the Origin of the Species.
What Reichert discovered was that two of the bones in our middle ear are the same bones that are parts of the jaw in reptiles. Now, that doesn’t seem to make much sense… how does a jawbone become a middle ear bone.
He learned this by tracing the resulting structures during development of embryos. He tracked what happened to those gill arches from the earliest development all the way through a completed organism. There was no mistake, what becomes a jawbone in a reptile becomes the middle ear bones in a mammal. They were the same. But without an evolutionary framework, how can two bones in two widely different organisms be ‘the same’.**
When paleontologists got involved with this line of research (1910-1913) they provided additional evidence. When you look at the very earliest of the mammal-like reptiles, you find one middle ear bone and a jaw composed of several different bones. But as you scroll upward through time, following the line that started with the mammal-like reptiles and as you go to successively more mammal-like organisms, you can actually watch the bones that become the middle ear reduce in size and change position. The jawbones had become middle ear bones, just like Reichert said they had 70 years before.
In this image, the ‘primitive synapsid’ at the bottom is the earliest mammal-like reptile. The top is the early true mammal. The colored bones trace the movement of the jaw into the inner ear.
Now, I knew most of this in a basic way for a long time, the details are pretty cool though. But what is really exciting is when I learn something totally new and it’s even more exciting when it is directly relevant to my life.
I have vertigo. My version is something of a failure to communicate between my inner ear and my eyes. Sometimes, when I turn my head, my eyes lag behind the motion of my head. This causes my ears to report movement and my eyes to not report movement. My poor reptile brain gets all confused and I fall over and spend the next two days in bed taking drugs. I also have a training program that I use every year or so to retrain my eyes to respond correctly to head movements. What this means for me is that roller coasters are right out and airplanes aren’t much fun (I can’t read on airplanes because of the difference between what my eyes are telling my brain and what my ears are telling my brain). I also don’t get drunk. ***
One of the effects of having a high blood alcohol level is that the inner ear system that keeps you balanced gets screwed up and you stagger around like an idiot for a day or so. The reason is that the alcohol from your blood stream goes into your inner ear canals and displaces the fluid that is used in your balance system. That movement of the fluid as it is displaced is sensed by the nerve cells and is reported to the brain as movement… when you aren’t moving.
The reverse happens the next morning. Your liver has removed most of the alcohol from the blood, and the alcohol in your inner ear slowly diffuses back into the blood stream. The inner ear fluid moves back to where it’s supposed to be and your ear reports that as movement too.****
That’s pretty dang cool and I didn’t know how that worked before. Of course, getting drunk may be a pleasant buzz for some, but for someone with vertigo it would be pure hell.
Now, back to the evolution. Guess what. This inner ear system is directly descended from sharks. A neuromast organ is a tiny little system that sharks use to detect movement of the water around them. If you’ve ever seen sharks suddenly jerk to one side and bite at a little fish, they were probably using that lateral line to sense the changes in the water flow as the prey fish swam by.
Those neruomast organs are basically the same thing as your inner ear. The same jelly-like substance in a sac with sensory hairs stuck into it. As the sac is compressed or moves from side to side by ocean currents (or you bending over), the fluid moves the sensory hairs and the shark can sense a fish and you can sense the change in your orientation.
All this goes back to the genes. This time Pax2 controls the development of the inner ear. A mutation here results in a badly formed or missing inner ear… perhaps I have a mutation there or (much more likely) an ancestor had a mutation there and I got it from my grandmother, who had horrible vertigo. Much worse than me… which makes sense evolutionarily speaking.
Now here’s were it gets REALLY interesting. Ready?
If you go all the way back in evolutionary history to the Jellyfish***** we find something amazing. The box jellyfish is a particularly venomous Australian jelly. It also has eyes, more than 20 eyes, in a variety of forms. The eyes range from simple pits with photoreceptors to complex eyes with a kind of a cornea and a nervous structure eerily like a human’s.
But the jelly doesn’t have Pax6. It doesn’t even have Pax2. Jellies arose before those genes developed. But when you look closely at the jelly’s genes, you find this amazing combination of Pax6 and Pax2. It’s a mosaic of primitive forms of these two genes.
So what, well there are two important points here. The first is that the with a primitive organism like a jelly having genes that are so similar to two of the genes in almost all the animals in the world, we get a glimpse as to how major changes can develop. At some point in the history of life, the jelly gene became to separate genes, each with a slightly different function.
Also, we see why so many human birth defects affect BOTH eyes and ears… from major problems all the way to my relatively minor case of vertigo.
To me, and because of my condition, these two chapters truly emphasize that fact that ‘nothing in Biology makes sense, except in the light of evolution’.
* There appears to be some evidence that some humans have developed a fourth opsin production gene, which allows a human with the fourth gene (invariably female) to see some 100 million unique colors. I am convince my wife has this gene, of course, she’s an artist and that would be an advantage for her. I have apparently regressed and have one 1 gene for color because I can’t discriminate between ecru and eggshell.
** So you see, the knowledge of common descent was based on work that was prior to Darwin. He only was able to combine all the disparate pieces into a coherent picture and he wasn’t the only one who could do so. He just published first.
*** For reference sake, I also loathe the smell and taste of alcohol. It’s truly disgusting in any form and I can put the entire volume of alcoholic beverages I’ve drunk over my entire lifespan into a 12 ounce coke can and have room left over.
**** I’ll also add that all of this is directly connected to the muscles that control your eyes. That’s one way cops can tell you are drunk. You keep shifting your eyes as if your head is moving, but it’s not moving, which means yours eyes suddenly jerk to the right as you are getting drunk (and soon after). After a night’s rest, your eyes will be jerking to the right as the alcohol leaves the inner ear.
***** Yes, I don’t like ‘jellyfish’. It’s not even a vertebrate, much less a fish. I try to call them ‘jellies’, but old habits die hard.