And we're back.

Refresher

I'm not gonna recap everything from all 7 episodes, but so far we have covered the domains, various clades and ranks and groups, grown multicellular and divided into 'bilateral' species, where each side of our body is symmetrical.

This part of the tree accesses all animals known as bilateria, but that still encompasses 1.3 million species, from insects to birds and indeed, us; our final destination.

So where do we go from here? A group called Nephrozoa. Honestly at this point I found it surprising there were still unfamiliar words for groups. I assumed it would all be 'mammal', ' primate' and so forth. So out of curiosity, I took a wrong turn and continued on to have a quick look what separates from us at the Nephrozoa level.

Nephrozoa

Oddly, this literally translates to 'Kidney animal'. This is particularly odd given that Neprhozoa are defined by their excretory system for the most part, since this is in the evolutionary tree where the excretory organs and nerve cords evolved.

For the most part, Nephrozoa splits into two groups:

Group 1: Protostomes

This is where I reared off to explore another direction, just briefly. Protostome translates as 'first mouth', referring to how their first embryonic opening develops into a mouth - before the anus develops. An alarmingly specific thing to define an entire group of nature on sure, but it might not surprise you to know that most worms fit into this category.

I came across some pretty unconventional ones, too:

• Arrow worms. A marine predatory worm that makes up a portion of plankton
• Penis worms. @trumpman's favourite animal.
• Girdle Wearers. A weird, microscopic animal that looks nothing like a wearer of girdles
• Mud dragons. Like a mud dragon, except a worm
• Hair worm. A really long, thin worm. Looks a lot like a roundworm
• Roundworm. Urgh
• Strange worm. A worm whose mouth is also its anus.

Some of these are worth entire posts on their own, but I don't want to go too deep in this direction. If we were to look at what else is here, we'd expect to see molluscs, snails, arthropods and those blasted tardigrades:

But let's take a step back from protostomes and look at something closer to home:

Group 2: Deuterostome

You guessed it; 'second mouth'. So what makes these so different from Protostomes? Essentially, what developed into a protostome's arse instead became our mouths, and vice versa. That's the fundamental difference in this grouping. Surprisingly, like every step of this journey, there have been unavoidable grey areas and exceptions exceptions; there are some protostomes that evolved 'deuterostomic' properties.

The class of deuterostome is indeed where birds, fish, mammals and all vertebrates reside. We are all arse-mouths.

This might seem like a huge portion of the diversity of life fitting into this group, but for perspective, before we broke off into these two groups we were still alongside 1.3 million species. After breaking off with protostomes, we are now only 78 thousand in number.

But that's good! It's getting easier to find us humans!

Looking at the tree representation, the first species we come across is... a worm.

Urgh.

An acorn worm to be precise. These break off with 8,000 or so other species alongside starfish and urchins about 530 million years ago, early on in the Cambrian period. At this point the remaining 70,000 are classified as chordates.

Some interesting terminology starts coming up here in the various branches of the tree: Olfactores, Craniata, Osteo. This makes sense because we're slowly approaching the development of bones and skulls - features like noses would be a consequence.

So 5-15 million years later, we all decided to get a backbone, and this is where another surprising choice feature separates us:

Jawed vs Jawless

Jawless vertebrates, or Agnatha are creepy as hell if you ask me. How on earth does one survive without a jaw? Allow me to demonstrate:

This is a Lamprey, a jawless fish. It's basically what it looks like, a spiral of death, a sucking mouth that's clearly the inspiration of many designs for horrifying monsters in games and movies. These creatures make good parasites, latching on to bigger things with their torture funnel to get easy access to the nutritious blood within. But they're not that small:

_{Poor thing. The fish, to be clear}

So, moving away from Jawless we get Jawed, or Gnathostomata. This translates to 'Jaw mouth'. The flattery in the term makes me blush. But the group comes with more than just a jaw:

In addition to opposing jaws, living gnathostomes have teeth, paired appendages, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheaths of neurons. Another is an adaptive immune system that uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene._Source

A little over 50 million years later, another split comes, and we're finally looking at pretty recognizable features you might expect to see in the zoo.

Bony vs Cartilaginous

Boneless backbones? What's going on here then.

Chondrichthyes, as they are called - translated simply from 'cartilaginous fish' includes sharks! The question here is 'how on earth did our entire skeletal structure decide to go one way or another; cartilage or bone?' This topic isn't nearly as simple as it might seem. This needs some digging to be done something I will attack in the next episode.

TL;DR

So, We've come a long way today, if only a few hundred million years. We've separated from 1.3 million species to a little over 60,000, evolving through bilateral symmetry, arse-mouths and mouth-arses, jaws, vertebrates and finally, bone. During this journey we've adopted some semblance of a skull, teeth, inner ears and adaptive immune systems.

I don't know about you but I'm starting to wonder all kinds of freaky things about my body.

All Images CC Licensed

References:

_{The developmental basis for the recurrent evolution of deuterostomy and protostomy(paywall) | Deuterostome | Protostome | Jawless fish: Lamprey | Gnathostomata}