Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body

by Neil Shubin

All

creatures with limbs, whether those

limbs are wings, flippers...

have a common...

but behind its throat were large vascular sacs: lungs.

Our upper arm has a single bone, and that single bone, the humerus, attaches to our shoulder. In the lungfish, we have a fish with a humerus. And, curiously, it is not just any fish; it is a fish that also has lungs. Coincidence?

We were staring at the origin of a piece of our own bodies inside this 375-million-year-old fish. We had a fish

with a wrist.

We are not separate from the rest of the living world; we are part of it down to our bones and, as we will see shortly, even our genes.

This patch of tissue was named the zone of polarizing activity (ZPA). Essentially, the ZPA is a patch of tissue that causes the pinky side to be different from the thumb side.

The DNA recipe to build upper arms, forearms, wrists, and digits is virtually identical in every creature that has limbs.

A great evidence for evolution. But creationists still use this evidence as an argument to independent creation from one creator. I do believe in God, but my belief is largely out of faith and not of scientific evidence. If anything, evolution does prove that God is omnipotent.

All appendages, whether they are fins or limbs, are built by similar kinds of genes. What does this mean for the problem we looked at in the first two 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

involved using ancient genes, such as those involved in shark fin development, in new ways to make limbs with fingers and toes.

When you see these deep similarities among different organs and bodies, you begin to recognize that the diverse inhabitants of our world are just variations on a theme.

Their reaction to the plumbing and wiring inside my wall was almost exactly like mine when I opened the human head and saw the trigeminal and facial nerves for the first time. The wires, cables, and pipes inside the walls were a jumble. Nobody in his right mind would have designed a building from scratch this way, with cables and pipes taking bizarre loops and turns throughout the building.

We’re all modified sharks—or,

Those insignificant-looking swellings and indentations have captured the imagination of anatomists for 150 years, because they

look like the gill slits in the throat regions of fish and sharks.

my surgical colleagues are operating on an inner fish that unfortunately has come back to bite us.

Funny remark about our similarity with other creatures--our origins.

The muscles and cranial nerves we use to swallow and talk move the gills in sharks and fish.

In seeing these embryos, I was seeing a common architecture. The species ended up looking different, but they started from a generally similar place.

Pander’s three layers gave von Baer the means to ask important questions. Do all animals share this pattern? Are the hearts, lungs, and muscles of all animals derived from these layers? And, importantly, do the same layers develop into the same organs in different species?

Von Baer compared the three layers of Pander’s chicken embryos with everything else he could get his hands on: fish, reptiles, and mammals. Yes, every animal organ originated in one of these three layers.

No matter how different the species look as adults, as tiny embryos they all go through the same stages of development.

There is still no evidence of the body plan. It is a far cry from this ball of cells to anything that you’d recognize as any mammal, reptile, or fish, much less a human.

Features such as the tube-within-a-tube arrangement are shared by all animals with a backbone: fish, amphibians, reptiles, birds, and mammals. These common features appear relatively early in development. The features that distinguish us—bigger brains in humans, shells on turtles, feathers on birds—arise relatively later.

Haeckel was comparing apples to oranges.

Biologically, Spemann had demonstrated that in the early embryo some cells have the capacity to form a whole new individual on their own.

Now, the million-dollar experiment: take the product of Noggin from a sea anemone and inject it into a frog embryo. The result: a frog with extra back structures, almost the same result as if the frog were injected with its own Noggin.

When did bodies arise, how did they come about, and, most important, why are there bodies at all?

Not every clump of cells can be awarded the honor of being called a body.

The cells inside the wart aren’t following the rules: they do not know when to stop growing.

One message from this is very clear: creatures with many cells began to populate the seas of the planet by 600 million years ago.

Evidence of these changes is seen not only in the fossil bodies but also in the rocks themselves. With the first bodies come the first trackways. Etched in the

rocks are the first signs that creatures were actually crawling and squirming through the ooze.

What could we humans, with all our complexity, ever share with impressions in rocks, particularly ones that look like crinkled jellyfish and squashed rolls of film?

But arguably the most important connection between cells lies in the ways that they exchange information with one another.

with some collagen interspersed.

Then, something surprising happened: the cells came together.

Many of the molecules that microbes use to cause us misery are primitive versions of the molecules that make our own bodies possible.

percent of our entire genome is devoted to genes for detecting different odors. Each of these genes makes a receptor for an odor molecule. For this work, Buck and Axel shared the Nobel Prize in 2004.

What makes them special is that impressions of the soft tissue, such as gills, guts, and the notochord, are often preserved.

These aren't usually found and the fossil record has only a few of them.

fossils of this quality are exceedingly rare.

Eyes rarely make it into the fossil record.

from simple photoreceptor organs in invertebrate animals to the compound eyes of various insects and our own camera-type eye.

we understand that we are simply a mosaic of bits and pieces found in virtually everything else on the planet.

Much of the processing of the images we see actually happens inside our brains:

Despite the stunning variety of photoreceptor organs, every animal uses the same kind of light-capturing molecule to do this job. Insects, humans, clams, and scallops all use opsins.

In these very different creatures—flies, mice, and humans—geneticists were finding similar kinds of mutants.

In a very real sense, they are the same gene.