Your anus is special

26 March 2015
Posted by Sara Sjosten.

You are special, unique, strange. As a human, some would view you as the crowning achievement of evolution. But to me, it's not your opposable thumbs that make you special, nor your nicely symmetrical body - even sea anemones have the genes to be bilateral.[1] It's your bum; or rather, your anus.

You may not have realised it, but your anus sets you apart from the vast majority of the animals that cover the Earth in a fundamental way. It also reveals that you have some surprising cousins in the tree of life.

Close Cousins

The tree of lifeIf I asked you to list a few animals, you might start with dogs, cows, tigers and pandas, maybe even throwing in a platypus or an axolotl to mix it up. But these are all vertebrates, things with a backbone, from the one branch on the tree of life to which humans belong. Think further afield to critters you spot on the beach or on your windshield and you will start to remember that there are a million species of animals on the planet: squids and jellyfish, oysters and lobsters, butterflies and centipedes, tapeworms and head lice.

While musing on the great diversity of animals, consider this: if all animals share a common ancestor that they evolved from, a great-great-great-grandmother of animal life, then one of these strange animal groups must be the closest relative of the vertebrates. This is where your anus becomes important. It tells us that you are the strange one, not the scarab beetle, and that there are only a few other creatures that are strange like you.

In the Beginning

Life starts with an egg and some sperm (or in some rare cases, an immaculate conception of sorts[2]). A newly fertilised egg is a single cell containing all the genes it needs to build an animal. The first step this one cell takes toward becoming an animal is dividing in two and then two again and again to make more cells to use as building material. These early cells move and change in a pattern that sets the stage for later development.

Until the embryo reaches a certain point, the way it grows is so fundamental that all animals do things pretty much the same way. A millipede, a panda and Jaws (either the shark or James Bond villain) are all pretty much indistinguishable in these first steps of life. One cell splits until there's a little, hollow ball of cells, which caves in on itself to make what will be the animal and placenta. A dent in the cell ball is where it caves in, and this indentation will become the mouth of the future animal.

Figure 1 - Development of a typical animal

This is where things get weird. Almost any animal out there develops this way: the protostome way. Proto- means first and -stome means mouth, so it's the mouth-first method. Around this first mouth-opening in the early embryo the animal develops into an earthworm or a mosquito or an octopus or hundreds of thousands of other species.

Humans do things differently, what could be called the anus-first method (officially known as the deuterostome way). This would-be mouth-dent instead becomes the anus, and a second mouth-hole appears later (deutero- means second, so mouth-second).

Hinging your development around your anus is actually quite a rare thing to do. While more than a million species are protostomes, the number of deuterostomes is at least ten times less. Humans are deuterostomes, as are all of the vertebrates. Further afield on the tree of life these animals become harder to find. As it turns out, our closest cousins are the only other two groups that put their anus first: hemichordates (worm like animals) and echinoderms (starfish and family).

Family Relations

Just about all animals fall into either the protostome group or the deuterostome group, only a few simple creatures like jellyfish and some worms are outliers. In recent years it has been popular to sequence the genomes of living things, and the genes of a wide range of protostome and deuterostome animals seem to show that the two groups have evolved separately, plus that each group is its own family. As a family, all the deuterostomes probably evolved from some shared common ancestor.[3]

The closer in the tree of life we move towards humans, the more complicated the family relationships are to disentangle. The deuterostomes form a small family, with only the three groups - vertebrates*, hemichordates, echinoderms - yet how these groups are related is still a mystery. Who is our closest cousin?

Figure 2 - possible relationships between phylums

Weird Creatures

While we may not know who exactly our closest relative is, one thing is for certain: all the options are weird. Looking at the non-vertebrate animals and making a guess, a reasonable choice might be insects because they live on land and have heads and bodies like us, or maybe an octopus that has arms and is smart. A sea-urchin would not make my list. Yet from the development of the embryo and the genetic code, it is clear that the crazy creatures of the echinoderm and hemichordate groups are our closest family.


There are five types of echinoderm alive today. The most familiar are the starfish and the similar brittle stars. There are also the pervasive beach nuisances, the sea-urchins; the squishy black sheep of the family, the sea-cucumbers; and the lesser-known sea-lilies and feather stars.

Orange starfish Brittle star Urchin Sea Cucumber Sea Lily

These creatures really look nothing like humans. Echinoderms have a five-fold symmetry (e.g. star and pentagon shaped bodies), have their nerves spread around their bodies instead of a brain, use water they pump through their veins to move, and make unique calcite crystals for use as plates of body armour. None of this is at all what humans or vertebrates do. Yet we know these creatures must be our relatives from the anus we share.


There are no common hemichordates, but there are two types nevertheless: acorn-worms and pterobranchs.

Acorn worm, pterobranch and hagfishAcorn-worms look essentially like burrowing penises (not to be confused with penis-worms[4]). They live in holes they make in mud on the seafloor and can grow up to eight feet long by filtering their food out of the water. Beyond this they don't do much, and looking at them it is plain to see that they have little in common with humans.

Pterobranchs are weird, the really weird kind of weird. They look more like an undersea plant than anything human. With bodies made out of tubes wrapped in mummy-like bandages, they extend long arms with feathery tentacles at the end to catch particles from the water. They are very rare and tend to root themselves under rocks for shelter, often in interconnected colonies.

Though very different to humans, these hemichordates do filter-feed through simple gills. Compare an acorn-worm to a simple old vertebrate* like a hagfish, with an eel-like boneless body and many gill slits through its neck, and with squinted eyes you may start to see a family resemblance.

But, at the end of the day, these are all peculiar. And none of them are particularly reminiscent of humans or lizards or fish. Specialist scientists argue circularly over the detailed anatomy and genetics of the group, trying to reach a firm decision on which of these bizarre creatures is our closest cousin, but there just isn't enough evidence.[5]

Luckily there are some reinforcements from the past.

Stranger fossils

None of the groups that make up the deuterostomes are new to the Earth. They have all been around in one form or another for a good half a billion years. And in that time they have taken many forms, some odder than others. Sometimes the weirdest fossils look so strange because they are combining the features of several types of modern animal into a monstrosity of sorts.

These monstrous chimeras may be the key to linking different groups of animals together - if an old fossil is half starfish, half tadpole, does that not hint that it may be the ancestor to both animals?

Ancient starfish?

Cothurnocystis elizaeThe nearly half a billion year old fossil creature Corthunocystis is one of these ancient oddballs that doesn't look like anything alive today.

It's shaped like a cup, with a skeleton made from the same kind of crystals as a starfish. But, it isn't star-shaped, and it has a series of small bones forming what could be a tail or a stalk, depending how you look at it.

While certainly strange, it's no easy task, even for a palaeontologist who is an expert on these animals, to determine what exactly they would have looked like. Having died so long ago and been buried in rock, most of the tissues that formed the body are long gone, and even the skeleton may have been broken or deformed over hundreds of millions of years underground.

Some scientists look at these remains and see a long tail made of vertebrae that would have protected a nerve cord, and a body with gills and a mouth-opening between starfish-like skeletal plates: This must be the ancestor to both the vertebrates and the echinoderms since it has the features of both!

But others are less convinced that this fossil really can show that vertebrates and echinoderms -humans and starfish - are closest cousins. They look at the little bones smeared in the rock and see none of the vertebrate features others claim to find.[6]

Headless fish?

Fossils of other creatures from the same time tell different stories of deuterostome ancestry.

Yunnanozoon was a worm- or eel-like animal swimming or wriggling through ancient oceans. Like Corthurnocystis, the fossils are old and the remains poorly preserved. This animal may have had a heart and gills like a vertebrate, or it may just have had general organ goop, more like a worm.[7] If it had a head maybe it was like an early vertebrate swimming around, if not maybe it was more like an acorn-worm. Does this fossil ancestor show that actually hemichordates and vertebrates are closest cousins? Or is it just too old and destroyed to draw any conclusions from its supposed anatomy?

Vetulicolians too are old wormy-fishy animals. They seem to have a mouth-opening at the front and gill holes in their sides and a muscular tail, but no head or limbs.[8] Some people who study these fossils think they see a spinal cord running down the back. Is this what the first vertebrate would have looked like? Does this mean that early vertebrates looked more like acorn-worms than starfish, and that pterobranchs are the closest cousins to us?

Yunnanozoon and Vetulicolia

Persistent mysteries

The conclusion reached is inconclusive and what we seem to know is a whole lot of nothing. Finding our closest family is almost like some murder mystery, "The Case of the Missing Cousin", and like any mystery case, what we need is the key evidence. As it stands, there is evidence galore from the anatomy and genetics of living creatures and fabulous fossils, hundreds of millions of years old, but the key evidence to solve the case is still missing.

What we can say is that you and your anus are strangely special, and your butt links you to a whole family of the weird and wonderful.

*The vertebrate group is essentially the same as the chordate group. While vertebrates have a backbone as a defining feature, chordates have a spine free of bone (called a notochord). The chordate group includes a very few non-vertebrate creatures, including eel-like hagfish and the bizarre sea squirts, which only show their spines as tadpole-like larvae.

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