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One of the central concepts in evolutionary theory is common descent, meaning that all living organisms can be traced back to a single common ancestor. In other words, all life on Earth is related.

That means, in theory, you could trace your way from child to parent to connect even the most distant of evolutionary cousins. And if you did, you would, at some point, come face to face with the very first animal. But what that animal would look like is… a good question.

There are clues all around us. They're hidden in layers of rock and in our DNA. And scientists have been piecing together these clues for centuries in the hopes of understanding exactly how animals came to be.

Sponges—those marine animals perhaps most famous for their use as cleaning implements— don't have a lot of traits that other animals do so it seemed pretty logical that the animal that came before all of us might have looked a lot like them. But as more and more evidence comes to light, the idea that sponges are our best link to our ancestors has become somewhat shaky. And that's led to a heated debate among scientists.

Some stand by the sponges. Others think mysteriously beautiful animals called comb jellies give us a better picture of our past. Today, we're going to unpack this fierce academic debate.

And all though we can't say which side is right, understanding the arguments for both does help us envision what the first animal might have looked like. Starting way back in the 1800s, pioneers of evolutionary theory like Charles Darwin and Alfred Russel Wallace began drawing evolutionary trees to map out how animals related to one another. And building evolutionary trees is still an important tool for understanding phylogeny – the evolutionary history and relatedness of different kinds of organisms.

Basically, each branch on these trees represents evolutionary innovation— a new trait or set of traits that separates the organisms on that branch from their closest relatives— while each stem represents a common ancestor. Figuring out this branching can be tricky. But, by paying close attention to the things that differentiate species from one another, scientists can reconstruct how they think organisms might have separated.

And when there are a couple different branching patterns that might fit what they observe, they tend to follow the principle of parsimony: basically, that the simplest explanation is probably the best. One neat thing with such evolutionary or phylogenetic trees is that you can use the information on different branches to basically back-calculate what traits a common ancestor likely had. And since the first animal is the common ancestor at the very base of the animal tree if we can construct that tree accurately, we can infer what that animal looked like.

Like Darwin and Wallace before them scientists today look at physical traits or morphology when mapping out phylogenetic trees but they also consider physiology, behavior, and genetics, and use computer modeling techniques to fill in the gaps. And while this has the potential to make the picture much more complete it can also make things a lot more complicated. You see, the first animal family tree was built with that parsimony principle in mind and assumed that the simplest animals were the oldest.

We consider ourselves to be pretty complex, and the fossil record confirmed that bony skeletons are a pretty new development, so it made sense that the phylum humans, hummingbirds and angler fish belong to—Chordata— should be the furthest branch from the tree's roots. Other animals that share a lot of traits with our branch —like having three main tissue layers and a body that's symmetrical when split down the middle— are the next branches down as you work your way towards that first animal. Then, way further down, you reach the point where jellies and their relatives in the phylum.

Cnidaria branched off. Cnidarian bodies are considered simpler than other invertebrates because they consist of only two layers of tissues with a kind of special goo sandwiched in between. And after them, there are really only two phyla left: the Ctenophora, delicate gelatinous predators often called comb jellies, and the Porifera, filter-feeders more commonly known as sponges.

Sponges are considered the simplest animals because they lack the complex structures found in most other phyla, like a digestive system or nerves. They don't even have true tissues— they've just got some gooey stuff sandwiched between two thin layers of cells, all of which is supported by tiny hardened bits called spicules. Compared to sponges, comb jellies seem downright complex.

Not only are their bodies arranged in a symmetrical way, they have muscles and nerves and the ability to capture their food with the help of sticky tentacles. In fact, they look a lot like the swimming stinging jellies which can ruin an otherwise delightful day at the beach. So it's generally been assumed they were somewhat close to their gelatinous kin. which leaves sponges as the first animal offshoot.

If this tree is correct, we can predict the ancestor of all animals probably looked quite… spongy. Like sponges, they would have had asymmetrical bodies composed of specialized layers of cells rather than tissues. And they would have probably stuck in one place, lacking the ability to swim around like comb jellies.

But . . . when scientists have tried to recreate this classic evolutionary tree with genomic data, they keep getting a weird result. The genetics often say those comb jellies are the lowest branch — at least, according to computer models of how DNA has evolved. Some scientists argue this is an artifact of genomic sequencing.

They think that ctenophores evolved really quickly— basically, their gene sequences changed more rapidly over time than those of sponges, and that's throwing off the evolutionary models that are used to reconstruct the big family tree. And that might be the reason why they appear to be closer to the common ancestor of all animals when you infer relatedness by examining genomes. But other researchers disagree.

Strongly. They say the math is sound and the placement of comb jellies as the earliest offshoot isn't an artifact. You might think the fossil record would sort all this out.

All you'd have to do is look back to see which group of animals started appearing first. But while we think of the fossil record as this neat, detailed log of life on Earth, it's more like your organic chemistry homework after the dog's had a go at it. The pages are wrecked, there's chunks missing, and if we're being totally honest, it wasn't complete to begin with.

That's because soft, fleshy things don't fossilize well, so their fossil record is patchy at best and that's before geologic processes wear down or completely erase what might little have existed. In fact, fossil ctenophores are so hard to find that the first was described in the 1980s. Still, some really great comb jelly fossils have been found the oldest of which dates back to about 520 million years ago placing it in the early Cambrian—the period of rapid animal diversification which began 541 million years ago.

Because of their hardened spicules, sponges have a fossilization advantage over ctenophores, so unsurprisingly, we have a lot more ancient sponges to examine. But, as a March 2018 paper in the journal Paleoworld points out there still aren't any clear sponge fossils from before the Cambrian. And something really interesting happens when you look closely at the sponge fossils we do have from that early Cambrian period when we know ctenophores were around.

These first sponge fossils share a lot of features with… ctenophores. As that 2018 review paper put it, their characteristics seem to put them on a continuum with the earliest known comb jellies— which could actually support the idea that comb jellies came first! There are slightly older fossils, dating back to about 600 million years ago, which some claim are sponges.

But they also have features which make them very un-sponge-like, so some have argued they aren't really sponges at all. So in the end, the fossil record—at least so far— doesn't do a good job of clearing things up. Which leaves us with looking at how these animals are now.

Comb jellies might look a lot like true jellies and their relatives, at the cellular level, they're vastly different. That's because they have a nervous system that's unlike any other animal on Earth. For instance, they have completely different neurotransmitters than the rest of us animals.

Everything from jellies to us uses serotonin, dopamine, and acetylcholine to communicate between neurons. But comb jellies use small proteins called neuropeptides as chemical messengers. So, if ctenophores were the first animal lineage to spit off then it's not entirely clear what came before them.

Some think that the common ancestor lacked any neural structure more like a sponge, and that comb jellies just happened to develop their own nerves independently. But that would imply that nerves evolved twice. And this is a really hard argument for a lot of scientists to swallow because of that whole principle of parsimony thing.

Neurons and neural communication is incredibly complex. And it does seem kind of far-fetched that something that intricate would have evolved in ctenophores and then again in the ancestor of everyone else. But, just because it's unlikely, doesn't mean it's impossible.

When species that are really different from each other develop similar traits it's called convergent evolution. And it's happened over the tree of life. Like, echolocation in bats and dolphins.

Or, the wings of birds and insects. Heck! Our eyes are extremely similar in form and function to the eyes of octopuses and squids, even though our last common ancestor occurred over 500 million years ago.

So if something as complex as an eye can develop separately in different species, the independent evolution of nerves doesn't seem that far-fetched. Or, maybe they didn't evolve more than once. It could be that the common ancestor to all animals did have a simple nervous system like comb jellies which was then modified a lot before the cnidarians split off.

Though, that may suggest sponges lost this trait. And that possibility raises an important point. Though it's sometimes or even often implied in evolutionary biology, the complexity of an organism doesn't tell you how old or how quote “evolved” it is.

Some argue that the assumption of increasing complexity causes us to overlook the fact that animals gain and lose traits through natural selection all the time. Parasites are a clear example of this—as they co-evolve with their hosts, they often lose things that their hosts can do for them. For example, this kind of reductive evolution can be seen in the genomes of Rickettsia— he pathogenic bacteria behind diseases like typhus that have evolved to live inside other cells.

They've straight up lost a little over a quarter of their genome, and a lot of what's left is scrambled nonsense. And we see it all the time when animals adapt to new environments just look at pretty much anything that lives in a cave. I mean, Mexican blind cavefish have completely lost their eyes!

The truth is, there's no reason that animals farther up in an evolutionary tree have to be more complex than those that are closer to that common ancestor. We might try to blame the idea that evolution equals complexity on things like the fossil record, since we see more complex animals in it than simpler, gooier creatures. But really, this idea that evolution produces increasingly complex things probably comes from the fact that we humans like to think we're pretty special, and we look for the traits of ours that set us apart from our animal kin.

The thing is, in the process of indulging in such biases, we risk taking for granted how amazingly complex so-called simple creatures like sponges and comb jellies actually are. I mean, comb jellies can generate light as a defense mechanism. Chemical reactions in their tissues allow them to shimmer magnificently in blues and greens.

Can you do that? And sponges aren't as simple as they might seem. One sponge species found on the Great Barrier Reef has over 40,000 genes in its genome— that's about twice the number you have.

Why does it need such a complex genome? We don't know. And when you think about it, getting past our own preconceived notions of how the world works is kind of the whole point of this debate.

Whether you're on team sponge or team comb jelly, it's the science that counts. At the end of the day, scientists care which animal is closest to our common ancestor because they want to have a richer and more complete understanding of our evolutionary past. It doesn't really matter who is right or wrong, what matters is that we aspire to get to the truth of how animals actually evolved – even if the truth looks different than we thought.

Evolutionary biologists will continue to debate how animals evolved for many years to come. Maybe someone someday will find some incontrovertible evidence that settles things— or, maybe they won't, because there just isn't any to be found. As long as they keep an open mind and continue asking good questions, all of us ultimately benefit, because the debate enhances our knowledge about the world we live in.

Questioning the status quo and digging for the truth about life on this planet is what biologists have always done. And I, for one, hope they keep it up! Thanks for watching this episode of SciShow!

If you liked this deep dive into the origins of animals, you might like our episode looking at the oldest fossils we've found and what they tell us. And if you want to be among the first to watch our newest episodes — about everything from fossils to physics — you can go to youtube.com/scishow and subscribe. [ outro ].