As a very cold period ended, huge glaciers that had formed. Moving slowly, they scraping nutrients from the Earth’s crust to be absorbed by the oceans. This led to a buffet of oxygen-producing algae, providing the oceans with a dramatic increase in living organisms over the next fifty or so million years. Sea levels were very high – maybe a third of mile higher than they are today. Temperatures are mild; the first primitive plants moved onto land, probably a form of the green algae from the water. Over time, that green algae transitioned to very large ferns. Conditions were right for some important steps to take place.
About 600 million years ago, a little animal appeared that acted like a fish but looked like a worm. So what? Remember the jellyfish with a strangely-arranged nervous system? Well – this little worm-like animal had a feature critical to our later development; a nervous system cord attached to a stiff but flexible backbone.
This little worm-like thing signals the development of an internal skeleton. At this point, no bones yet; instead, up the back, a stiff but flexible bone-like structure which has four flexible bony connections to the fins which will eventually become a bony spine with connections to the four limbs. Equally important, as the stiff part becomes our backbone, this step will carry the spinal cord to the brain. In time, segments of muscle, sort of like a string of pearls, wrap around that stiffened segmented part, providing protection.
Paying attention? Your backbone started to develop with a cord to the brain just like yours.
That worm-like animal evolved into a truly weird one – a fish without a jaw. Jawless fish, making their debut in both in ocean saltwater and freshwater lakes, had a fairly rigid backbone. That internal bone had a sort of upside down arch appearance; a like a couple of McDonalds arches.
The animals were ugly but their senses complex. They could sense sound and pitch and had eyes. Bottom feeders without a jaw, they had a sort of throat underneath them that sucked in water, took out the what was needed, and sent it back out through the gills. Actually, the stiff part was more like cartilage then bone; quite flexible. Some, but not all, scientists view jawless fish as the first vertebrates.
Paying attention? They could hear; they had eyes; and they had a throat.
All around them were bigger, stronger invertebrates who viewed the jawless fish as breakfast, dinner, or snack. In response, over time, these jawless fish developed a tough armored plate on the head. As bottom feeders, armor protected them from attacks from above. In time, that water intake hole moved around to the animal’s front leading to a jaw. These animals, by the way, were TRULY ugly. Say “hello” to the placoderms, not only ugly but nasty. They had an “open wide” mouth, sharp teeth, and were covered with protective armor – sort of an armored medieval warrior with weighting in at about a ton. Placoderms. In one bite one could cut a shark in half! Eventually they went extinct, but attention is needed; they were important contributors to the evolutionary line eventually leading to us. Well, at least humans did not inherit that big heavy plate on their heads.
An animal with a flexible backbone followed the placoderm. The flexible bone is called cartilage, so (ta-daaaa) they are called the cartilage fish; the modern-day shark is the best-known cartilage fish. A your next visit to an aquarium (or being attacked by a shark) watch how smoothly they move through the water. The flexible backbone allows that movement.
When no one is looking, reach up and hold the divider inside your nose with your thumb and a finger. Move it around. That divider is cartilage, given to you before the cartilage fish spun off from placoderms. Another example: the sports page today reports on an athlete with a cartilage tear in the knee. All of us have cartilage bone junctions. And this will really shock you – but it is true: while you were in your mother’s womb, safe and warm, your earliest bones were cartilage. Then they changed to bones. So the cartilage fish took off on their own but before doing so made some contributions to you.
Besides the cartilage fish, a second group split-off from placoderms had backbones without that flexibility. The bones became hard so (ta-daaaa) they are called “bony” fish. By this point, all science agrees; these are vertebrates, the line that led to amphibians then reptiles and finally mammals and us.
Paying attention? Your bone structure has been added.
So while the cartilage fish went off their merry way, the bony fish line itself split into two parts. One group of bony fish remained in the deeper water.
The first group are what we still call “fish.” Between the oceans and fresh water lakes and rivers, as many as 25,000 fish species from the bony fish line are spread around the globe. For a Minnesota-born fisherman like the author, catching walleyes remains a pinnacle event—almost the reason for living.
The other group sought safety from those huge salt-water animals. The other pathway moved into shallower fresh water, in ponds, bays, and little streams caused by very high water levels on earth. This group needed a place where those huge saltwater predators could not go. This line was called the lobe finned fish because their structure was bones and muscles. Funny name; but this animal is in our direct evolutionary line.
OK, folks. Your ancestors moved out of the salt-water oceans to clean, fresh water. Feeling better?