Evolution 101: The Family Tree of All Animal Life on Earth

 

All Life on Earth is Connected

Whether you’re a marine worm or a marmoset, the exact same genetic code proliferates your DNA. Our molecular biology exposes a common ancestry that dates back four billion years, almost as old as the Earth itself. But how can bacteria and bonobos be cousins? How can human intelligence have arisen from brainless invertebrates? The answer is a truckload of time… plus the natural process of evolution.

 

What is Evolution?

Evolution is the emergence of new species through random genetic mutations. That’s right – evolution is primarily fuelled by random chance, by mistakes in DNA replication. So how do these mistakes translate to being better adapted to survive and reproduce?

This is where Darwin’s theory of natural selection comes in. Depending on what type of environment you live in, mutations in your genetic make-up can have a positive, negative, or neutral effect on your chances of survival. Mutant camels who can store more water in their humps survive better in the desert. Mutant beetles with more effective camouflage are less frequently picked off by predators. It all comes down to the relative success of mutations in a forever changing environment.

Mother Nature is a bitch. So even the smallest mutations can have life or death consequences. Organisms who survive longer are more likely to reproduce and pass on their mutant genes. And disease-causing mutations have the opposite effect. Over time, beneficial mutations spread through the generations and become the norm across a population: the “wild type”. That is, until further mutations and environmental pressures shape new adaptations and speciation events. You end up with a planet full of different species, each adapted extremely well to their own environmental niche.

Simple, right? Actually, it’s a hell of a concept to get your head around. So the rest of this article is dedicated to explaining evolution in the context of animals: a single sub-set of multiple domains of life.

The Three Domains of Life

Biologists have grouped all life on Earth into three domains: Domain Bacteria, Domain Archaea and Domain Eukarya. This wasn’t an arbitrary choice. Genetic analysis tells us they each arose long ago from a common single-celled ancestor with poorly developed genetic machinery. There’s no fossil evidence of this guy (he’s pretty old, and pretty small) but comparative genetic analysis of his living descendants is a powerful method for understanding the history of life today.

 

Three Domains of Life - Bacteria, Archaea, Eukarya

So animals are just one sub-set of life on Earth. But they are the most familiar, which means tracing the ancestry of animal species is a pretty cool way of understanding how evolution works. Starting from a single-celled eukaryotic ancestor, here’s how life evolved into the great diversity of animal species we see around us today.

When Life Went Multi-Cellular

Animals were born 770 million years ago when a type of single-celled organism (a “choanoflagellate”) began living in colonies. This new adaptation to stick together was beneficial and saw the rise of new collective organisms called Porifera, or sponges to you and me. You may not think of sponges as animals. They have no brains, eyes, blood, organs, or even true tissues. They are merely clumps of cells which have become somewhat specialised to function in complementary ways.

Porifera - Sponges

Originally, sponges were realised as basal animals by the fact that free-living choanoflagellates are virtually indistinguishable from the body cells of sponges. More recently, genetic analysis has verified that they share a ton of common genes with animals alive today. Roald Dahl had it right when he named Aunt Sponge; these are indeed your most distant animal relatives in the world.

Radial Symmetry

Let’s jump forward 100 million years now, which is about the time it took for the next major developments in animal body form to emerge: soft bodies and radially symmetrical forms.

 

Cnidaria - Jellyfish

Cnidaria – a group of animals including jellyfish, corals and hydras – boasted distinct tissue layers and an internal cavity for digesting food and transporting nutrients. They also began displaying radial symmetry: bodies which have a top and bottom, but no left or right sides. Like a pie, they can be sliced up equally along many axes and still look the same.

Remember that these guys evolved gradually. Beneficial mutations to their body form and behaviour accumulated over millions of generations until they could no longer reproduce with true sponges, which is what classifies them as a new species. Multiple new species emerged at different times. And not all descendants of sponges eventually evolved into Cnidaria, which is why sponges still exist today.

Bilateral Symmetry

Over millions and millions of years, aquatic animals diversified into many exotic forms. Flatworms evolved from something like hydra, some 550 million years ago. These wormy fellas boasted a new feature called bilateral symmetry: having a distinct head end (“cephalisation”) with mirrored right and left sides.
The significance of this may not be clear until you consider the lifestyles of radial vs bilateral animals. Radial animals are usually immobile (think sea anemones) or drifters (think jellyfish). Their all-round symmetry means they can meet their environment equally well from all sides. Bilateral animals are active movers (see the flatworm), encountering their environment head-on where all the sensory equipment is located. The flatworm also possesses a central nervous system, with bundles of nerves (“ganglia”) concentrated in its head; an early, if primitive, brain.
Platyhelminthes - Flatworms, Flukes

Exoskeletons

If you were to go outside and pick up a handful of soil you would encounter thousands of our next guest: Nematoda. These squiggly guys are equally content living underground, in the ocean, or in your gut. Their evolutionary super power is a tough outer coating (a “cuticle”) which serves as an exoskeleton to protect their squishy inner organs. They are highly adaptive but pretty simple creatures and that is precisely what makes them so successful.
Nematoda - Roundworms

Arthropoda include a million species of insects, spiders and crustaceans which account for 80% of all animals on Earth. In other words, arthropods rule. Like nematodes, they feature an exoskeleton which they must moult (and secrete a new one) in order to grow into their adult form. They also sport jointed limbs and segmented bodies, both of which can be highly specialised. For instance, a lobster’s appendages are specialised into sensory antennae, pincers, mouth-parts, walking legs and swimming appendages. Each species have different combinations suited perfectly to their environment, a process driven by survival of the fittest, also known as natural selection.

 

Arthropoda - Insects, Spiders, Crustaceans

 

The Cambrian Explosion

Like many of today’s living animal phyla, Arthropoda first appeared during the Cambrian explosion, a time of rapid evolutionary change 525-535 million years ago. This is kinda cool because we can relate massive environmental changes (over a few million years) to the explosion of new life on Earth. It’s a wild demonstration of evolution by natural selection. So what caused the Cambrian explosion?

  • Environmental factors point to increased oxygen levels and receding glaciers. An essential fuel for metabolism, more oxygen meant animals could grow larger and pursue more energy-intensive lifestyles, such as hunting. A fall in glaciation also meant more light could penetrate the oceans, and as you well know, sunlight was essential for the aquatic plants to thrive and become food.
  • Ecological theories suggest co-evolution pushed predators and prey into a race for survival. For instance, predators developed adaptations to swim faster, while prey evolved hard protective shells to make them less susceptible to being gobbled up.
  • The genetic explanation is perhaps the most compelling: “Hox” genes underwent key refinements. Hox genes are the master control switches for body plan development, so that a minor mutation in a Hox gene can create a massive difference in the resulting animal. In a short space of time, Hox mutations saw many developmental variations tested out by nature.

With the Cambrain explosion in mind, let’s explore some more major animal phyla (and their accompanying novel features) that emerged on Earth some half a billion years ago.

 

Segmentation

Over in wormsville, Annelida evolved, including the deliciously slimy earthworm. Named for their segmentation, each body chunk is internally and externally identical to the next. Some annelids possess paddle-like feet which also function as gills. Why gills, you cry? Because many annelids are actually marine critters, drifting in the ocean or burrowing into the sea floor. Some are tiny: less than a millimetre. Other annelids are behemoths: consider the Giant Australian Earthworm which can top three metres. Others suck your blood, like leeches. Mmmm.
Annelida - Earthworms

Shells

Sometimes when you walk along the beach you find a really nice shell. As a child I always thought shells were naturally occurring fancies but no, they are in fact the calcium carbonate shells of Mollusca.
Mollusca - Snails, Clams, Octopuses
Despite first appearances, molluscs are actually a really interesting and diverse phylum. Their special features include soft bodies which excrete a hard protective shell for, you know, survival and stuff. Most molluscs are marine (clams, oysters, snails and squid) although some live in freshwater or on land. Wait a minute, Columbo, did you say squid? Yes, sir, indeed I did. Squid have a small ancestral shell present (a “gladius”) but it’s reduced and internal. It supports the squid’s mantle and serves as a point for muscle attachment. Now there’s something most people don’t know.
Take a moment now to open Google Images and type Cephalopods which is the collective name for octopuses, squid and chambered nautiluses. Oh never mind, I’ll do it for you. These are extremely intelligent invertebrates who independently evolved eyes and, in the case of colossal squids, grow up to 14 metres in length. They are incredible and beautiful creatures.

Deuterostomes: A New Mode of Embryonic Development

Did you know that you have quite a lot in common with starfish? Protest as much as you like, but DNA comparison reveals that sea stars, sea urchins and other Echinodermata are sisters to all living “chordates” (a group we’ll cover exclusively from here on along our evolutionary tree of animals, including the very familiar animals like crocodiles, dogs, birds, zebras and, oh yes, humans).
The striking similarity between a starfish and yourself is your “deuterostome” development. Back when you were eight cells small inside your mother’s Fallopian tubes, you took on a special mode of “indeterminate” cell division. This means that if we isolate a single cell from an embryo at this early stage of development, it can go on to divide and form a complete human being. This, incidentally, is how identical twins are possible, and where stem cells are in abundance. The alternative to deuterostome development is called “protostome” development, and was the only mode possible in all animal phyla seen thus far. Here, the fate of all embryonic cells are sealed early on.
Besides their surprising deuterostome development, echinoderms boast unique features, such as an internal water canal system branching into tube feet which together enable movement and feeding. Like sponges, sea stars may not look like animals, but there are very much living creatures which breathe, eat and reproduce, either sexually (with sperm and egg) or asexually (breaking in two and regenerating).
Echinodermata - Starfish, Sea Stars, Urchins

Chordates

As I teased just moments ago, we now move into the evolutionary territory of chordates. These are animals with a nerve chord (which develops into the brain and spinal chord), a flexible notochord (the start of a backbone), “pharyngeal” slits (which function as gills or ears depending on your species) and finally, a post-anal tail (typically for swimming, though it is much reduced in humans).
The earliest chordates are lancelets: little blade-like critters which burrow backwards into the sand, leaving just their mouthparts exposed to catch passing food particles.
Cephalochordata - Lampreys
Next up are the sea squirts, or Urochodata. You may also know them as tunicates. The larval form, which may last only a few minutes, is a tadpole-like creature, swimming in search of a substrate on which to attach itself. Like you and I, it has all the chordate features. But then it undergoes a radical metamorphosis. It “eats” its own brain. Major anatomical features – including the tail, notochord, primitive eye and rudimentary brain – are absorbed into the body. It will spend the rest of its life immobile, siphoning water through its body and extracting food particles as it goes.
Urochordata - Tunicate

The Vertebrates

The next major development in animal evolution is represented by Myxini, or the hagfish: a jawless vertebrate with a skull made of cartilage. Take a look at this beauty queen.
Myxini - Hagfish

The hagfish is a slime-producing marine animal that, while incredibly ancient, still exists as a number of different species today. It has rudimentary vertebrae, a brain, eyes and other sensory organs. In evolutionary terms, that’s kind of a big deal.

Moving on, this next shocker is a face for radio. Lampreys, or Petromyzontida, are other early adopters of backbones, and let’s not forget, is your great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-grandmother. Just saying.

 

Petromyzontida - Lampreys

Jaws

Next came the evolution of jaws and a mineralised skeleton. Welcome to the age of true predators. Chondrichthyes (namely sharks and rays) may be a mouthful to say but he will gladly have a mouthful of you in return. Being able to eat big chunks of flesh enabled sharks to grow bigger and swim faster compared to his ancestors. Jaws was indeed an apt movie title.

 

Chondrichthyes - Sharks, Rays, Skates

 

Lungs

You may have noticed that we’re still in the ocean along our evolutionary trail. That’s because our sea animals have not developed any capacity to gulp air. Until now. The ray-finned fish, Actinopterygii, diverged from its ancestral line carrying the unique trait of manoeuvreable fins and a swim bladder. For many fish, the swim bladder is a buoyancy aid; an air-filled sac which keeps them at their water depth without having to waste energy on swimming. It is also a rudimentary lung.

Actinopterygii - Ray Finned Fish

We can’t leave fishy territory without acknowledging the lobe-fins, or Sarcopterygii. The most famous of which is the coelocanth, thought to have gone extinct along with the dinosaurs 65 million years ago. In fact, they were discovered to be alive and well when, in 1938, a fisherman hauled one in and took it to local naturalist, Marjorie Courtenay-Latimer. “I picked away at the layers of slime to reveal the most beautiful fish I had ever seen,” she said. “It was five feet (150 cm) long, a pale mauvy blue with faint flecks of whitish spots; it had an iridescent silver-blue-green sheen all over. It was covered in hard scales, and it had four limb-like fins and a strange puppy dog tail.”

Sarcopterygii - Lobe-Finned Fish

Tetrapods

By now, the most interesting life forms on Earth had eyes, a brain, a backbone, jaws, gills, lungs and muscular fins. It required only small modifications of this body plan to evolve into tetrapods (“tetra” means four; “pod” means foot). The 2006 discovery of a species called Tiktaalik provided the missing link between the lobe-fins and land-dwelling tetrapod animals.

Like a fish, Tiktaalik had fins, gills and lungs, and its body was covered in scales. But unlike a fish, it had ribs to ventilate its lungs and support its body out of water. It also had a neck and shoulders to help move its head. Perhaps most significantly, Tiktaalik’s fin-feet had the bone structure common to tetrapod wrists today. This extraordinary creature would give rise to the first tetrapod land dwellers: Amphibia.

 

Amphibia - Salamanders, Frogs, Caecilians

While amphibians are comfortable on land, their lives are inextricably connected to the water. Their moist skin and eggs are vulnerable to drying out, so they can never truly explore the far reaches of dry land. Nonetheless, they’ve been a successful group of animals which includes all frogs (including toads), salamanders, and caecilians (legless snakes). They go through quite the metamorphosis early in life, having larval forms which are distinctly different from their adult forms.

Amniotic Eggs

If gooey eggs tied early amphibians to the water, then hard-shelled eggs would liberate their descendants: a massively diverse phyla called Reptilia. I’m going to spend a disproportionate amount of time looking at reptiles now because they include the most popular animals that ever existed: dinosaurs, birds, snakes, lizards, crocodiles and turtles. They evolved over millions of years, of course, and some are much more closely related than others. What might surprise you is that birds are indeed classed as reptiles and their closest surviving relatives today are the crocodiles.

 

Reptilia - Snakes, Lizards, Crocodiles, Birds

Consider Testudines, the bizarre shelled-reptiles known as turtles, tortoises and terrapins. These are seriously ancient lizards: the oldest sea turtle fossil dates back 120 million years, so it lived alongside dinosaurs, and its body plan remains virtually unchanged today. In evolutionary terms, that means it has not accidentally discovered (for mutation is random) any better adaptations for life in the ocean than it already had 120 million years ago. That tends to happen with really ancient life forms.

Testudines - Turtles, Tortoises, Terrapins

A completely different evolutionary branch among Reptilia, dinosaurs lived between 230 and 65 million years ago, a time collectively known as the Mesozoic Era. We split dinosaurs into two major groups. First up, Ornithischia (or “bird-hipped”) were mostly herbivores named for a pelvic structure similar to that of birds. Over the course of evolution, some ornithischians developed armoured plates and thick skulls which protected them from predation by carnivorous dinosaurs.

Ornithischia - Ankylosaurus, Parasaurolophus

The other group of dinosaurs were the Saurischia, which included beastly fellows like T-Rex and Giganotosaurus; massive docile long-necks like Diplodocus and Brachiosaurus; and smaller feathered critters like Archaeopteryx. Their collective name translates to “lizard-hipped”.

Saurischia - T-Rex, Diplodocus, Archaeopteryx

Aves, or birds, are thought to have descended from saurichian dinosaurs, although recent research suggests we still need to shuffle the evolutionary tree a little. What we do know is that birds evolved some remarkable bodily adaptations to aid flight, which has massive survival benefits: hunting, escaping predators, and widening their territories, to name a few.

Hollow bones, lack of teeth, a single ovary in females, and specialised feathers all help to get birds off the ground, where so many dangers lurk. Take a look at native New Zealand birds, who evolved in geographical isolation for 80 million years and didn’t have to worry about predators. Many species, such as the kiwi and kakapo, are flightless, so when 19th-century Europeans arrived with cats, rats and dogs, flightless bird populations crashed. Flight is one of the single best survival adaptations full stop.

Aves - Blackbirds, Ducks, Parrots, Keas

We have now briefly considered all modern day animal phyla except one: Mammalia. By definition, mammals are animals which have hair and produce milk from mammary glands. I don’t know my dad’s excuse. Fossil evidence shows how the jaw was gradually remodelled in early pseudo-mammals, over a period of 100 million years, and alongside the dinosaurs. By the end of the Triassic period they were mostly small, hairy creatures which fed on insects at night and probably still laid eggs. They diversified to a degree, but the competition was fierce: dinosaurs already dominated many ecological niches.

The three lineages of mammals – monotremes (egg-layers), marsupials (pouches) and eutherians (placentas) – were already established when large dinosaurs went extinct. The mammals that survived them soon exploited their habitats, food sources and territories, rapidly filling the gaps left by dinosaurs.

 

Mammalia - Monkeys, Rhinos, Llamas, Bears

This brings us to the present day. From choanoflagellates to chameleons, life finds a way. Yes, I’m quoting Jurassic Park. It seems wrong not to. Jurassic Park was a sharp-toothed demonstration of how animals will ruthlessly exploit their environment to survive.

In fact, the majority of life that’s ever existed has struggled and fought not to die. Many perished in the jaws of some larger animal or at the mercy of famine or disease. These are the challenges which shaped the evolution of all animals, including man. Until now.

Are We Still Evolving?

David Attenborough suggests that, for the first time in billions of years, we may have stopped evolving. Sure, we still make mistakes in our DNA replication and this creates novel mutations. But our species now wields considerable control over our environment so that the effect of natural selection is barely felt. Agriculture ensures a steady food supply. Vaccines ensure herd immunity against infectious disease. Criminal justice ensures the weak aren’t preyed upon by the strong. Gene editing and other cutting-edge technologies even offer a direct method of altering our genetic code, paving the way for designer babies and permanently altering our germ lines.

For better or worse, we have taken our evolution into our own hands. We have decided that all human life has value. The smarter ones have even figured out that all animal life has value. Mother Nature can no longer kick us out of the gene pool when she feels like it. Instead, we will swim to our hearts’ content, few of us pausing to realise that billions have lived and died before at the mercy of evolution.

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