I can smell the end of this series right around the corner...

Credit: Brocken Inaglory CC 4.0

Recap

Last time, we reached a symbolic landmark of evolutionary complexity: The placenta. We learnt that, as special and unique as it seems to be, it's actually quite a common and seemingly inevitable. It becomes increasingly apparent that this is a common theme in the history of Evolution

But from here on out, what could possibly be so transformative to make us humans unique from the rest of the animal kingdom?

Eutheria

Amazingly, the answer at this stage in the evolutionary tree is still not so clear-cut. There are a number of proposed clades, or groups, that are more or less prominent depending on who you ask. For example, Atlantogenata is a proposed clade that separates animals such as sloths and anteaters from the rest of us on the other side of this branch, called Boreoeutheria, both coming from Eutheria (Placental mammals). But these come with little rhyme or reason or at best, weak proposals not nearly adopted by the masses.

So we're going to skip through a few million years until we hit about 55 million years ago and visit the very first:

Primates

Astonishingly, right up to primates there are still no particularly distinctive features among any particular grouping of animals that taxonomists ubiquitously agree upon.

But primates are clearly different to non-primates, so what makes us unique? Big brains, opposable thumbs, heavy reliance on vision over smell? Almost anything you can consider uniquely primal can be found elsewhere.

BIg Brains

In short: Dolphins. (The brain is obviously the most complex subject in the Universe so I'm not going to tackle that just yet)

Opposable thumbs

This is something some people think is unique to humans. This is very clearly not the case; gorillas, chimps and so on to start, but opposable thumbs aren't even limited to primates. Take the tree frogs Phyllomedusa, for example, a family in which not only do some frogs have opposable thumbs, but fingers, too. This is a great example of convergent evolution; these tree frogs live in trees, surprisingly, and have adapted opposable thumbs to allow a tight grip to navigate around.

For this reason, Opossums also have them on their back legs to complement their prehensile tails up in the trees.

More peculiarly, Giant Pandas also enjoy opposable thumbs to help them strip bamboo up (since that's literally all they do). But in the case of the Panda, their thumb is actually a freakishly overgrown carpal bone; one of the wrist bones beneath our fingers that actually grew outwards to become a kind of 'false thumb'.

Weird Panda Thumb Credit: Ali karimifard CC 3.0 _{It says share-alike but I felt the translation was necessary so whatever}

_{This fact actually allowed some clarification in the evolutionary timeline of Pandas, disconnecting them somewhat from Red Pandas thought to be a recent ancestor. Both pandas have opposable thumbs, but the common ancestor of the red panda was arboreal (tree-dwelling) and evolved the thumb for this reason. Giant pandas, it turns out, evolved it totally separately in 'one of the most dramatic cases of convergence among vertebrates.'Source. This means that pandas actually evolved thumbs twice, even though the red panda is more closely related to ferrets.}

Anyway, my point is, our thumbs are certainly not unique. So what else?

Vision

This is where it gets interesting. We very much take for granted the history that goes into the vibrancy most of us live with and enjoy every day, so let's correct that because it really is unique to primates.

Tetrachromacy - Public Domain

Back in the day, some vertebrates had tetrachromacy, or four types of cones to pick up colours. But for reasons likely to do with our history of a nocturnal lifestyle, mammals typically dropped two of these cones and left most modern mammals with dichromacy or even monochromacy. What makes us primates unique is our evolution of a new, third cone, making primates trichromatic.

The ability to see more colour would have helped us distinguish, say, ripened fruits and with little biological cost, so it stuck around. But it gets much more complex and interesting than that. One hypothesis suggests that trichromacy was selected for:

... discriminating the spectral modulations on the skin of conspecifics, presumably for the purpose of discriminating emotional states, socio-sexual signals and threat displays_Source

This also, according to the hypothesis, notes that trichromatic primates are typically bare-faced to help with these discriminations.

Understanding Trichromacy

You see, not all pigments are alike, though they function the same. Pigments reside within a membrane of our cone cells. By exciting these cells, we get the perception of colour sent to our brains. That's the shortest way I can think of summarizing it.

The first thing we need to understand is where this conical blueprint comes from. The Small pigment cones absorb light at around 430nm - blueish. These cones seem to originate from our L Chromosome and are ubiquitous in vertebrates suggesting a rather more ancient lineage.

Trichromacy - Credit: Vanessaezekowitz CC3.0

But the other two pigments - medium (530 - greenish) and large (560 - yellowish) come from our X Chromosome, one of the well-known ones that determine our sex. This is quite strange, but it explains why men are more likely to be colourblind than women since we only have one X chromosome.

You may notice that the distance between the medium and large wavelength is much smaller than between those and the small wavelength, and this sensitivity difference is no accident. What appears to have happened is the gene coding for pigments in the X chromosome mutated and duplicated, with the second one sensitive to a slightly different range of light.

So now primates have one pigment gene in the L chromosome, and two pigment genes in the X chromosome.

BUT NOT ALL PRIMATES

This rule, bizarrely, only applies to Old World Monkeys - Including humans. The New World Monkeys got it quite different. Only female new world monkeys have trichromacy... And even then only 2/3rds of them. All males have dichromacy.

Credit: Maphobbyist CC 3.0

You see, upon separation, isolating themselves in the Americas, new world monkeys such as spider monkeys and howler monkeys had their own play around with those X chromosome genes. The chromosome itself only allows for one slot of pigment genes like any other dichromatic animal, but a mutation created 3 alleles, or variations of this pigment; medium, large and a third one somewhere in between the two.

What happens next is the chromosome randomly ends up with one of those three alleles. For men, this assures dichromacy; with only one X chromosome and one L chromosome, they are limited to one blue pigment gene and one of the other three pigment genes - nothing more.

For the females though, they have TWO X chromosomes, each randomly selecting one of the three larger pigment genes. This means they get an extra opportunity to enjoy an extra pigment gene - but it's not guaranteed. Being random, there is a 33% chance that both X chromosomes choose the same gene, and they end up with dichromacy regardless. For the other 66%, they get to see the world like no other new world monkeys can.

There's more

Mantis Shrimp ain't all that. Credit: Alexander Vasenin CC3.0

The problem is, cones in our eyes are not little brains, and receiving the light is not enough alone to fully appreciate the colours that we see. This can be seen in mantis shrimp who famously took the internet by storm with their fantastic colour vision as a result of having 12 colour photoreceptors. But although this is true, they lack the brain capacity and energy to actually put these to use in the way we imagine. In fact, humans are far more capable of distinguishing between colours, with mantis shrimp practically guessing between to wavelengths 25nm apart and humans being capable of distinguishing 1-4nm apart. Clearly, they just use photoreceptors differently.

So how do primates old and new get to appreciate these colours so acutely? Surely in the case of the New World, if only 2/3rds of one gender in new world monkeys can see these colours, there'd be no need for the brain to develop some hefty tools to be able to comprehend them?

The research seems to suggest that, surprisingly, we already had the right tools for the job. The neural pathways responsible for the longer wavelengths are not actually dedicated to colour vision, but spatial vision. There is likely no separate network circuitry specifically for colour vision, the body just kind of hacked a previous installation, and not even one that recent either.

A study in mice genetically engineered their X chromosomes to have the duplicated X genes that we humans have. These mice then went on to be capable of distinguishing three colours that regular mice would otherwise find at least two of them identical. They could just... see it, no questions asked.

Plot twists

• Remember the New World Monkey, the howler monkey mentioned above? They are actually an exception in that both males and females have trichromacy.

• Remember how I said it was unique to primates? I LIED. As it turns out, some marsupials may evidently be trichromatic - and bees, too! Bees actually have an ultra-violet pigment gene alongside blue and green

So once again, we're not entirely unique in this, but our specific form of trichromacy is at least of its own kind. There's much more to it, but that can wait for another day.

Convenient Delegation Links:

References:

Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas | Atlantogenata | How the Panda's Thumb Evolved Twice | The Evolution of Primate Color Vision | Cone topography and spectral sensitivity in two potentially trichromatic marsupials, the quokka (Setonix brachyurus) and quenda (Isoodon obesulus) | Bare skin, blood and the evolution of primate colour vision | Fruits, foliage and the evolution of primate colour vision | Mantis Shrimp Vision Is Not As Mindblowing As You’ve Been Told