The First Animals: Weird and crunchy

Animal Evolution: Weird, Crunchy and Crabs

Slimy, scaley, great and small;
they are precious creatures, one and all.

People like to think they know a fair bit about animals. They come in different types, fish, reptiles, mammals – some eat plants, others eat each other. They are the part of nature we connect with most easily. However, the origin of animals is much more complex than many people would think – a lot of the early animals are really weird. And many of the most important steps in animal evolution are not the ones you. Let’s dive in, to animal evolution.

Animal Evolution: The stages

We don’t really need to give a definition of animals. They are probably the one group of organisms that everyone immediately understands – the big multicellular creatures we see moving around us. They include the common categories everyone learns at school: fish, amphibians, reptiles and mammals. However, they also include earlier and more simple creatures that people don’t immediately think of when they think animals: things like coral, sea anemones, starfish and sponges.

For animals to go from the most basic bacteria and microscopic creatures (see our posts here) to the glorious species we know today a lot of evolution occurred. The main steps were:
– evolving a spinal cord – the core of a lot of animal structure;
– evolving a jaw – to be able to chew and eat hard things; and
– evolving to lay eggs away from water.

The Earliest Animals: The Edicarian creatures (560 million years ago)

These are some examples of the “Edicarian Biota” – a strange set of small weird starfish-like animals that lived deep on the bottom of the ocean 500 million years ago. They are considered the first animals (Image: Paleozoo).

The first animals were pretty similar to starfish and coral. It’s easy to understand why, they are very simple. They don’t have complex sensory organs like eyes or structures like arms and legs to catch prey. Instead, they just sit on the ocean floor, absorbing nutrients as they float past.

These animals evolved 500 million years ago. There were not any plants at this time – the only other living things were single-celled bacteria and sludge. They pretty much had the planet to themselves.

Most of these distinctive early animals, called Edicarians, have gone extinct (except a recently discovered patch in Mexico).

These animals look a bit like trilobites, the most famous early animals. However, the Ediacaran animals came around almost 100 million years earlier than trilobites. Trilobites are only really famous because Darwin loved them and everyone used to think they were the oldest animals on earth. It’s Edicarians who were the original animals.

The Animal Shape: Getting a plan together

Most of the famous and well-known animals (fish, reptiles, mammals etc) have a spinal column. Their spine is very important for their structure – everything on us connects to our spine. However, there are a lot of animals that don’t have a spine. These are pictures of ‘non-cordates’ animals without spinal columns – the top six are different sea slugs (they’re pretty, we got carried away, sue us), the bottom two are a sponge and a starfish (Images: Wikimedia).

To build an animal, you need to have a plan. Animals grow in a pre-determined structure and shape, it’s what differentiates them from other creatures like plants and fungi. Plants grow much more randomly. For example, if a bit of a tree feels okay on one particular day, it will put out a shoot and that will eventually turn into a branch if all goes well. Different trees of the same species can be all sorts of different weird shapes, depending on all sorts of environmental factors. Not so with animals. A pig is pig-shaped, no matter what the wind was like the first few days after the pig was born.

Building a pre-planned animal shape is really hard. Imagine you need to build a car. But as you build the car, it needs to be attached to you the entire time (you are pregnant with the car). And you still need to be able to move around, eat and escape predators – a tiger can still go for you any moment. And you need to build the entire car together. You can’t just build the wheels separately, then attach them to the body. You need to grow the wheels and the body together. And, the only materials you have are the basic organic things you can eat. And it needs to work immediately, perfectly, first time or the animal dies.

So how do you grow a consistent animal shape? How do you build a complex structure, but all together, at once? The general idea is you grow a bunch of cells, and then turn different cells into different types of tissue. The outside cells turn into skin, bits in the middle turn into bones and organs, and bits in the middle turn into your spine. But how does the body do that? How do the cells know what to turn into?

To grow a consistent and complex animal shape, there needs to be some signal to tell different bits of the animal what they are going to be (i.e. skin, muscle, bone etc). This is done by what are called “signaling molecules” created in the first cells of an animal’s development. Then these initial cells split into more cells, as the animal grows. This means there will be more of this molecule near the center of the organism, and less of it further away. So basically a cell goes “oh look, I’ve only got a tiny bit of this molecule near me, I must be near the outside, I should become skin”.

To build a complex structure like this, an animal needs a centreline. Some central strip that all the biology is built on, around which an animal is mostly symmetrical. For the well-known animals (fish, reptiles, mammals etc) this central strip is their spinal column. It is their core, around which all their other structures (limbs, skin, teeth) are built. Animals are generally separated into these two categories – chordates, animals that are built around their spine and non-chordates, animals without spinal cords that have some other form of structure.

The animals without spinal columns, tend to be arranged very differently and be very weird. These are things we’ve already talked about, like starfish, but also mollusks (snails and octopuses), sponges and worms. A lot of these creatures are weird and not well understood. For example, sponges can be chopped up in a blender, and then if they are poured into another container, they can completely reform themselves with no damage within a few hours. Some of these worms can be chopped in half and just suddenly form two new worms. And some of the sea slugs are really pretty (see the above pictures).

Skeletons: Building rigidity

*A diagram of the two approaches to skeletons that animals have come up with (Image: Wikimedia).*

Hagfish – probably the first animal with a backbone. They don’t have much else, and this allows them to be super flexible and tie themselves in knots. They live deep on the ocean floor, feasting on the leftovers of ocean predators (Image: University of Guelph).

The next step, after being able to build an animal body, is building one that has some strength. A lot of the early animals tend to either be completely floppy (sea slugs) or maybe with a shell (snails).

To have a really effective structure, an animal needs to have both – hard bits to give it a shape and exert force on the world, but soft bits to be flexible and adaptable.

Animals have evolved two different strategies – internal skeletons and skeletons on the outside. Animals can generally be divided into these two categories “vertebrates” like fish, reptiles and mammals and “invertebrates” like insects, spiders and crustaceans.

The animals with internal skeletons all have a very consistent structure. They’ve all developed a skin to protect them from the outside world. They’ve also developed a series of other types of tissues (muscle, tendons, ligaments etc) to anchor the soft stuff to their skeleton and move it around. It’s pretty consistent with them all, fish, reptiles, mammals – they are all basically meat bags wrapped around skeletons with some muscle to move it all around.

The other animals – insects and crustaceans, developed exoskeletons. They moved around using hydraulic pressure, pumping fluid around their skeletons, in exactly the same way that modern hydraulic machines work. This means they are vastly stronger – ants can lift thousands of times their own body weight and survive falls from very great heights. It’s hard not to be a bit jealous.

A Jaw: Getting a proper meal

The first animal with a jaw was a weird extinct species of fish called Dunkleosteus. It was basically a crab shark – it had a hard exoskeleton like a crab but it was shark-shaped and swum around eating other fish. Its jaw evolved from one of the plates on its head. It was also probably one of the biggest animals at the time, it was three meters long and was probably one of the very first recognizable predators (Image: J John, Wikimedia).

A jaw is important for animals because it lets you eat complex crunchy things. Previously, most animals had some form of suckers or gills that really only allowed animals to eat small morsels of organic gunk that were floating through the water.

The first animals with jaws were likely a group of amazing, extinct animals called placoderms. They were the first predators that we’re able to eat and hunt other animals. The best way to describe them is probably as crab fish. They were fish-like, and swum and hunted animals. But they were also covered in hard bony plates, one of which eventually turned into a jaw bone they could use to eat other fish. There is nothing even vaguely like them on the planet anymore – they were really unique to this early period of life.

Onto the land: How to survive without water?

A mudskipper – one of the strange intermediate animals that is still around that lives on both water and land (Image: *Kombipom*, Flickr).

It’s not difficult to imagine how animals started to live on land. Over time, as the oceans got more crowded, fish would wash up onto beaches and graze on the small plants growing on the shore. These fish did pretty well for themselves, there’d be a lot of food with no competition. Over time fish started specializing in this, their fins becoming more feet like and their gills getting more lung like. Over time they probably started living in rock pools and isolated bits of water, spending almost all their time outside it but coming back regularly to breed.

This is basically what amphibians are. Frogs, salamanders (people are nowhere near as excited by salamanders as they should be – some massive metre-long ones still live in Japanese and Chinese rivers). They could probably have populated a fair way away from the ocean, and gone up into streams and out into the Earth.

The real change is when animals were able to reproduce away from the water. The problem with being an amphibian is that you still need to stay pretty near some water. And you basically still lay fish eggs, they need a lot of water, they’d dry out on land. So you’re not really a land animal yet.

The first animals to work out how to lay eggs are what are called “amniotic animals” – basically animals that lay eggs that are waterproof. Lizards lay an egg with a thin layer called an “amniote” that will mean their embryos develop in a wet environment, even if they are layed in a desert. This was the moment life truly came out of the oceans and into the land. From here, reptiles turned into dinosaurs, dinosaurs into birds, and mammals also happened. But that was all easy, the main change was building the animal shape and skeleton and coming out of the water.

Frogs eggs vs snake eggs. Frog eggs are much closer to fish eggs, they still need to be in the water to develop. One of the major steps in the evolution of life was animals evolving an “amniote”, a thin waterproof film around their eggs that let them reproduce away from water. From these animals, reptiles, dinosaurs, birds and mammals all evolved (Images: Wikimedia).

Animals: So what?

Most animals are actually invertebrates – roughly 97%. We like to focus on animals that are like us, but really, we are a tiny fraction of life the animal kingdom. And that doesn’t even include the bacteria, plants and fungi that are out there. Life on Earth is so much bigger than us, and things like us (Image: Oregon Institute of Marine Biology, University of Oregon).

People think they know animals. They think lions and tigers and monkeys – the big ones we see in nature documentaries. However, the animal kingdom is much more varied and glorious than this. It includes weird sea slugs, strange worms, and many weird early invertebrate things that we just don’t have simple English names for. We spend so much time researching ourselves, with medical research, and the animals we eat to improve on our food production and conserving the occasional large wild mammal that we think is cute (looking at you pandas). However, life on Earth is so much bigger than that. Roughly 97%of the animal species on Earth are actually invertebrates, most of them being insects. The big animals, the famous ones we know, are a tiny sliver of the animal kingdom.

This focus on the famous animals, also skews our view of animal evolution. We normally think about it in terms of amphibians turning into reptiles and then into mammals. And these are all important stages. But the really big steps in animal evolution are often not the ones we think of. To go from a starfish to a lion requires so much complex biology. Spinal cords, skeletons, jaws and waterproof eggs are the really fundamental shifts in animal life. Once you get one little four-legged thing walking around on the earth, it doesn’t take that much to get bigger, wackier variations of complex four-legged things. It’s the weird early animals, that really matter. We should value them so much more.

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