Paleontology is all about discovering the history of natural life on our planet and there's not much further back we can go beyond looking at what the very first animal on planet Earth was!
Of course, this is an incredibly broad and difficult question to answer, but we're going to try in this post!
Problems standing in our way of the answer
Answering the question of 'what was the first animal on Earth' comes with some hurdles to overcome. First up is being very careful with what we call the discoveries. Being the earliest fossil we have discovered does not equal the very first organism, it simply means nothing before it fossilised that we have found so far. This could be for a variety of reasons, covered in the post looking at what a fossil actually is, but when we are looking at organisms from before when hard body parts had actually evolved, the main issue we have is things being able to fossilise in the first place. This is why the Cambrian explosion is such a controversy, since we are still unsure whether a rapid evolution took place or it just looks that way since this is when organisms first started fossilising (also known as preservational bias).
This issue of preservational bias is one that is constantly on the minds of paleo-ecologists, since it can paint a very different picture from the truth.
What is an animal?
Next we need to bare in mind what actually constitutes an animal, since life goes back billions of years before animals were even a thing on planet Earth! Originally, life on Earth was made up only of single celled organisms who fed via photosynthesis, the same way as plants. Eventually, these cells evolved to find it much more beneficial to live together all snug inside a bigger cell, helping it function in exchange for shelter and food. This symbiotic relationship soon evolved into the other kingdoms of multi-cellular life including plants, animals and fungi. The animal kingdom is an incredibly diverse one, made up of varying degrees of complexity. However, there is one thing that unites this whole kingdom and one thing that makes an animal an animal.
The digestive tract.
Yep. Everything else, such as lungs, hearts, limbs, eyes and brains are all optional extras. Even coral counts as an animal, since it has the animalian digestive system (i.e. organic matter goes in through one hole, energy is absorbed and organic waste leaves through another hole).
Again, it seems pedantic, but these technicalities matter when we're talking about the first animal, since thinking nothing came before it would make it seem like animals just popped into existence (a concept reserved for religion and there's no way I'm opening that can of worms).
So, what was the actual first animal?
Well, originally, paleontologists believed that that the first animals to exist on our planet were sea sponges. This is thanks to their basal and relatively simple anatomy, being nothing much more than an organism with some internal hard parts and a simplistic digestive tract. Given sponges sedentary lifestyle, they would also require relatively little oxygen to survive, something that was in much shorter supply 800 million years ago. The only questionable part of this theory is the fact that sponges do in fact have a type of internal skeleton, something that, again, wouldn't have just popped into existence. This kind of inference was only concluded from scarce fossil evidence and phylogenetic study (a type of research which tries to computationally draw connections between extinct life and modern relatives).
However, a re-analysis of this data showed something very interesting back in 2008. Combining the previous data along with extraordinarily complex genetic studies, scientists were able to see that the earliest animal that we can see wasn't actually a sea sponge at all.
It was a comb jelly...
What is a comb jelly?...
So, what is a comb jelly and why is this such a fascinating revelation? Well, comb jellies are animals from the group formally known as Ctenophora. This group is actually a phylum level group. To put into context how big that is, one type of phylum is the chordata, a group that includes vertebrates...yes includes! A group at phylum level (which is only one level of classification below a kingdom i.e. animal kingdom) is so big that we vertebrates don't even make up the whole group!
But I digress.
Jellyfish are not included in this phylum, despite the name and how superficially similar they appear. Comb jellies are marine invertebrates that use cilia to swim (a type of tiny tentacle normally reserved for microscopic organisms) and have a jelly-like body made up of two layers each only two cells thick. These animals can be up to 5 feet in diameter and are usually predators, preying on other animals as large as tiny crustaceans!
So why the confusion about this being the earliest animal that we know of? Well, for a start, Ctenophores have surprisingly complex physiology, sporting connective and muscle tissue, a nervous system and even sensory organs. Again, these things couldn't have just popped out of no where, so what came before that gave the comb jelly these gifts? Then there's the fact that they are mostly predators. Surely (if they were predators back then) there were other critters to feast on? Of course, this one is clutching at straws, since nothing is to say these animals haven't flitted between predators, herbivores and filter feeders several times over the millennia, but it's still an interesting thought to process.
Having said all of this, it is important to stress yet again that the earliest known animal does not mean the very first animal.
Another point to remember is that this conclusion was reached from a phylogenetic computational study (AKA techy stuff looking at organism relations) and not hard fossil evidence, so the study has had its fair share of doubters. We will only truly know if said evidence turns up, until then, be sure to comment what you think of the findings!
Until next time!