The Origins Of Vertebrates Part 2: Acting Like Animals
Okay, we’re moving out of the realm of the single celled organisms this week! After mitochondria became a staple of complex life our eukaryotic ancestors diverged into fungi, protists (previously known as protozoa), the archaeplastida (algae and plants to you and me) and the gloriously underrated slime molds that I won’t get into right now. Did I forget to mention animals? Well, we probably emerged from a group of protists called choanoflagellates, a colonial family that don’t really look animalian in the sense that you’d think of today. But from these colonies grew a whole brouhaha of creatures. Let’s follow them and see if they can grow a spine.
So our ancestors probably looked something like this, be prepared to have your mind blown, they’re pretty ugly and technically single celled, but probably became so dependent on each other that they began to function as one organism until that was indeed what they were.
Gorgeous isn’t it?
Animals appeared during the Precambrian period from these strange stalked colonies in a one off event that (by definition I suppose) was never to be repeated. Some of the first animals discovered were colonial and looked like leaves, obviously God testing us, and as animals grew more diverse and began to fill the many niches available to them the first hard parts begin to appear in an event we call the Cambrian explosion. We call it that because it’s in the Cambrian period and in geological time a whole load of creatures seem to emerge in a really short space of time. We don’t call it that because there was an actual explosion, that’d be pretty difficult to accomplish under the sea.
Now, it’s tempting to assume that this sudden appearance of many fossils meant that animals were just taking off and making a foothold, but because hard parts (skeletons and the ilk) fossilize more easily than the soft parts that most creatures are made from, more animals with shells etc. are going to fossilize than the multitude of animals that lack them. So there might have already been a huge diversity of life that didn’t have a chance to fossilize, but what we do know is that creatures with hard parts did diversify rapidly.
So, you may ask, what are the major changes that occurred during this time? Well, there are a number of fairly major splits in the history of animals that bring us from the first animals to backbones and the rest of our animalian cousins. Let’s go over them briefly and discover what shaped the body plans of animals.
1. Parazoa and Eumatozoa
Parazoans lack ‘true tissues’ such as muscles and nerves, have a skeleton of silica and an outer cell body with many pores. You may know them as sponges. The rest of the animals? Well, they’re all pretty much Eumatozoan.
2.Radiata and Bilateria
Radiata include jellyfish, corals and hydras, as you might have guessed by the name they’re radially symmetrical, the same all the way around. They’re also diploblastic, which means that when they’re just beginning to develop they have two cell layers surrounding the cavity we call the gut (the endoderm and ectoderm). In Bilateria we find a third cell layer, the mesoderm, that exists between these making them triploblastic and allowing for for complex tissues and organs to be developed. They include us, as you might suspect, seeing as we’re not incredibly symmetrical, and their bilateral symmetry allowed for more localised organisation of nerves. Whereas in jellies there’s a nerve net, in Bilateria the nerves group near the mouth in a primitive head.
3. Acoelomates and Coelomates
Acoelomates lack the body cavity known as the coelem, that allows the gut to move independently of the body wall, these include flatworms that digest via diffusion into their gut through their body wall. Us Coelomates needed to work out a different way of getting nutrients into our body. You may have noticed that you don’t absorb oxygen through your skin for example, or at least not enough to keep you satisfied.
4. Protosomes and Deuterostomes
This is probably my favorite of these splits because it explains that we have an asshole where our mouths should be.
You heard me.
This split is largely determined by how cells go through cleavage (get your mind out of the gutter) in early embryonic stages. Protostomes include molluscs, annelids (such as earthworms) and arthropods (insects, crustaceans and arachnids). These go through spiral cleavage (not so easy to make it dirty now, eh?) and the cells specialize early, and Deuterostomes go through radial cleavage which leaves many cells unspecialized (hello stem cells). We belong to the latter group, along with sea stars, and whereas the initial opening in embryonic development becomes a mouth in Protostomes… well, ours becomes the anus and our mouth is, well, secondary.
Next time we’ll go deep into these Deuterostomes and I’ll apologize for it taking me until the third post in this series to actually get to a good spinal cord.