Anna Bishop here again, answering an awesome biology question from Ava Hartshorn, Age 7: How Were Animals Created?
( a picture of animal cells under a microscope)
Well, it all started about 575 million years ago, a time called the Ediacaran Period, when primitive animals began to develop. Lumps of cells probably weren’t what you were thinking of when you thought of animals! But lumps of cells created the first animals on Earth. There are three educated guesses about how this happened.
- The first thought is that animals were made from clumps of cells (the smallest units of life) that banded together (like a slime mold) in a group called a “grex,” and helped each other survive. Did you know a slime mold can move around, and is a kind of animal?
- Another idea is that one cell’s center separated into many cells, and a thin skin (called a membrane) formed around each one of these clumps and protected them. This group of cells could have become an animal.
- The third idea (and my personal favorite) is that when the “children” of one cell did not separate from their “mother” cell, the cells all morphed into a single unit. Each cell developed a special job to make work easier for the whole creature: This is seen in both animal embryos (animal babies that are not yet born) and in small cooperative cells that we see today, as well as in fossils from the time animals developed.
Every group of animals evolved from the ones before it. Have you heard of Darwin’s theory of Natural Selection? Darwin suggested that animals change and develop based on how well they are able to survive. Here’s an example: A giraffe with a very long neck will be able to eat the leaves from tall trees, but a giraffe with a short neck will starve to death because it cannot reach and eat the leaves. Since only the longer-necked giraffes are around to have a family, they will have babies that are like them: long-necked. In this way, every animal species gradually becomes better and better at surviving, and is more complex.
But back to the beginning days of animals. Let’s fast forward a few million years. Non-moving creatures that looked kind of like this (a Dickinsonia) began to appear. Exciting, right?
This animal most likely crawled along the sea floor, eating microbes (teeny cells like bacteria and protists). Many scientists think that animals were simple back then because there was not a lot of oxygen in the atmosphere. More complex animals like fish and insects need a lot of oxygen to stay alive!
The Earth was cold in those days, but eventually it began to thaw, and entered a period we call the Cambrian Period. Most of the early creatures like Dickinsonia died because more advanced animals evolved and began to eat up the microorganisms that the Ediacaran animals relied on for food.
33 millions years after the beginning of Ediacaran animals, we have what scientists call the Cambrian Explosion. A new animal came to be: Treptichnus pedum, a worm-like animal with a hook-shaped tail that burrowed in the floor of the ocean. The animals we know came after it looked sort of like snails and clams: soft and slimy inside, with hard mineral shells to protect them. Most of them were only around two millimeters long, about the length of the tip of a crayon. Here is what some of their shells looked like:
Is this what you were thinking of when you asked about animals? Not yet? The next big change in animal life gave way to sponges and coral. Then came Trilobites, a group of animals related to Horseshoe crabs. These creatures moved, but didn’t run.
During the Devonian Period, organisms like fish, leeches, and starfish began to appear. In fact, one of the largest fish species ever to live on Earth lived during this time: the Dunkleosteus, an armoured fish that grew up to 33 feet. Its jaws had a crushing power of 1,664 lbs., and the fish itself weighed about 8,000 lbs.! To us, it would have looked like a sea monster! Late in the Devonian Period, amphibians (animals that lived on both land and water) and land-dwelling insects began to appear. Amphibians and reptiles evolved from lobe-finned fish, like this:
Next, during the Carboniferous Period, came spiders, scorpions, and mites. Reptiles grew popular in the dry conditions the Earth saw then. In fact, there were many more kinds of reptiles back then, and their sizes ranged from six feet to six inches. Animals like dragonflies began to develop, but not like the little pretty ones you see in your backyard. These dragonflies had wingspans of around three feet -- about as big across as the average 3-year-old! A scorpion-like animal, the Pulmonoscorpius, was a fearsome two and a half feet long. Many scientists think that animals were so big back then because a great amount of oxygen had developed in the atmosphere -- 35% -- greater than it had been before and has ever been since. Here is what an Archaeothyris, a reptile from which all mammals evolved, might have looked like:
At the end of the Permian Period, there was a disastrous mass-extinction. Almost all the ocean animals died, and more than half of the land animals died. It took the ecosystem more the 30 million years to recover. However, once the ecosystem had recovered, the age of the dinosaurs began: the late Triassic, Jurassic and Cretaceous.
Here’s where we can see some of the animals you might be thinking of: dinosaurs, the first birds, and the first mammals. The first mammal we know of is Adelobasileus, and the first dinosaur we know of is Plateosaurus. Here are artists’ pictures of what they might have looked like:
(Above) The first mammal (Below) The first dinosaur
Imagine that: all the dogs, horses, rabbits, and people you see today came from that fuzzy gray animal! The first mammals were Marsupials, like koalas and kangaroos, who carry a tiny baby around in their pouch until it is fully developed. This was because the early mammals’ hips were too small to give birth to a fully developed baby.
Next is an artist’s drawing of Archaeopteryx: the first bird. It was about the size of a raven, had teeth, and a long tail, which changed as birds evolved. The fact that it has feathers tells us it was warm-blooded, and therefore not a dinosaur, which is cold-blooded. Since it had sharp teeth, we know it was a carnivore, a meat eater. In this picture, it is chasing a bird-like dinosaur.
You might recognize some of these dinosaurs: Allosaurus, Stegosaurus, and Diplodocus. All of these were Jurassic Era dinosaurs, large and complex reptiles that had split into groups of carnivores and herbivores (meat eaters and plant eaters). There are very few dinosaurs known who were omnivores, (both plant and meat eaters, like us). Stegosaurus, one of the most recognizable of herbivorous dinosaurs in the Jurassic period, was a large, armored dinosaur, that protected itself from predators with a spiky tail. Even though it could be up to 30 ft long, its brain was about the size of a walnut.
During the Cretaceous Period was when we saw the Tyrannosaurus, Velociraptor, Triceratops, three of the most famous dinosaurs. Most mammals at this time were still small and mouselike, but had developed wider hips, making it more likely for their young to survive. The Earth was dominated by dinosaurs, the biggest and most powerful animals on the planet. It was the next era, the Paleogene, where mammals began to become many species, and the dinosaurs died out (We aren’t sure why, but scientists have a few ideas.).
Humans, as we know them, developed in the next period, the Neogene, about 7.5 million years ago. We were most closely related to Chimpanzees when we became our own species. Our most important trait to distinguish us from our ape relatives is walking on two legs, which other apes do occasionally, but not constantly, like we do.
We then have the Quaternary Period, the period we are in now. During this period, we domesticated (or, made into pets or farm animals) animals like goats, dogs, cats, horses, cows, sheep, and pigs. We moved around the world and became the largest mammal population on Earth!
The rest is history! I hope that answers your question, Ava! I’m glad to hear you’re curious about Biology, the science of life. Bye for now, and stay curious!
About the Author: Anna Bishop is a sophomore at Sturgis Charter Public School and critter enthusiast. She is part of the Teen Advisory Board for the CSF, and her interests include zoology, zoo-psychology, ethology, raising insects, linguistics, and trying new recipes.