Do you know?

I decided to pick out some of interesting facts from the book "Your Inner Fish" by Neil Shubin.


Limbs and Genes
A gene, named Sonic Hedgehog, is involved in the sculpting of the limbs of all tetrapods (which are homologous structures). Despite being so different from tetrapods, cartilaginous fish (such as sharks and skates) can be induced with the same effect by a mammalian version of the Sonic Hedgehog protein. I think Neil's conclusion of that particular chapter says lot, so I'm going to quote the book directly:

...What does it mean for the problem we looked at in the first 2 chapters - the transition of fish fins into limbs? It means that this great evolutionary transformation did not involve the origin of new DNA: much of the shift likely involved using ancient genes, such as those involved in shark fin development, in new ways to make limbs with fingers and toes.

But there is a deeper beauty to these experiments on limbs and fins. Tabin's lab used work in flies to find a gene in chickens that tell us about human birth defects. Randy used Tabin's lab discovery to tell us something about our connections to skates. An "inner fly" helped find an "inner chicken," which ultimately helped Randy find an "inner skate." The connections among living creatures run deep.

The History of Teeth
The hardness of our teeth and bones is the result of the mineral hydroxyapatite within the tissue matrix. This material's history in the bodies of animals stretches back to the ancient jawless fish, Conodonts, in the oceans 250 to 500 million years ago. These fish had no hard bones - Teeth came before hard bones. That first hard body parts arose to eat other creatures rather than for armour for defense.

Ostracoderms were fish with bony-head skeletons. These critters had a head region made of a bony plate like an armour. Looking at the bony armour under the microscope comes a surprise. It is made of thousands of fused teeth - a version of teeth was commandeered by evolution to be used as armour.


Whales and Smells
Olfactory receptor genes may be classified into two categories - smelling in air and smelling in water. Not surprisingly fishes have water-based ones and terrestrial animals such as mammals and reptiles have air-based ones.

Here comes the interesting bit: The transition from land back to sea is written down in the genome of cetaceans (whales and dolphins). These creatures no longer use their nasal passages for smelling. They have been modified into blowholes for breathing only. The olfactory receptor genes for air-based smelling in their genome are all present but knocked out by mutations. The selection pressure has disappeared for these genes as their function is no longer critical for survival.

What about humans? 300 of our thousand olfactory receptor genes has been knocked out by mutations. Quoting the book:

...humans do have a sense of smell, so why have so many of our odor genes been knocked out? Yoav Gilad and his colleagues answered this question by comparing the genes among different primates. He found that primates that develop color vision tend to have large number of knocked-out smell genes. The conclusion is clear. We humans are part of the lineage that has traded smell for sight. We now rely on vision more than on smell, and this is reflected in our genome. In this trade-off, our sense of smell was de-emphasized, and many of our olfactory genes became functionless.

About balls...
Here's something interesting about guys.

Do you know that in sharks, their testes are way up inside their bodies - above their liver and close to their heart? Guess what, even in humans, the testes of males were there too. It is a relic of our evolutionary and developmental history. When males are still developing in their mother's womb, the testes make their descent from above the liver and down to the scrotum. Because of this, the sperm cord takes an absurd route - from the testes up to the waist, over the pelvis and then down to the penis.


There's much more interesting stuff in the book of course and with more detail. Some are just too long or complicated to summarize here.