Today we continue our journey to humanity through the tree of life, piece by piece. We've traveled from the big three domains of life, all the way through some very grey, impossible to conclusively define areas that scientists still squabble over to this day:

• What are humans?
• Archaea, Bacteria & Eukaryota
• Unikont or Bikont
• Opisthokonta to Holozoa

But after all that, we're still 1.3 billion years in the past! The next phase is sub-billions of years ago, called the Filozoa.

Filozoa

Around 900 million years ago, we break off once more into a branch that still consists of 1.3 million known species. We're in there somewhere. Filozoa is defined as a grouping within the Opisthokonta, which is a broader group within the Eukaryota Domain.

It consists of Animals - Us - and unicellular relatives that aren't quite animals, but closer than that of fungi and others. The way things are organized here differ once again depending on your source, but typically its divided into Filasterea - which is the proposed name for the group consisting of various things that eat bacteria, and Apoikozoa - which holds true animals as well as choanoflagellates which I briefly discussed in the previous episode.

This means that the choanoflagellate you see on the right is the closest relative possible to 'Animals' - but not an animal. On the whole, we know of barely 125 choanoflagellates that exist today, all in various marine environments up to 300m in depth.

They do indeed serve an important role in the ecosystem, despite how we've never actually heard of them, by unlocking forms of carbon life couldn't otherwise access by chomping on bacteria, and giving higher organisms a chance to access it. This makes them a small yet vital player in the carbon cycle game.

Apoikozoa is literally translated to mean 'Colony animal', referring to how animals and choanoflagellates can be multicellular, like a colony. This means that those in the Apoikozoa are the only organisms to consume carbon for energy - heterotrophs - that form colonies of multiple cells.

However, Choanoflagellates only sometimes do this, raising another grey area in the tree of life; did they adopt this ability from further back in the tree before animals and they split? Or did animals and choanoflagellates develop multicellular abilities separately, and if so, what mechanism made it likely for this convergent evolution to occur twice?

How Animals came to be

This is one of the big questions. Perhaps I should reword it: How Heterotrophs became multicellullar. We're not at animals yet, exactly.

The simple answer is 'we don't know'. But there are many ideas. According to Graham E. Budd and Sören Jensen., single celled life would have formed kind of blankets of organic life which was common back in the day, competing for nutrient rich areas. These blankets would presumably come together and be the main link between single celled organisms and ediacaran life - the earliest known complex, multi-cellular organisms.

_{A classic example of an immobile blanket of complex life - an Ediacaran}

The Ediacaran period was a large one, spanning from 630-540 million years ago, the period that came before the Cambrian, though Budd & Jensen later put a much earlier date on their hypothesis of blanket-animal creation and Ediacaran life survived a good 50 million years into the Cambrian.

One striking theory is that Ediacarans actually evolved brains and nervous systems independently from the animal kingdom, yet still do not fit into animalia because they lack any embryonic stage of life. Martin Glaessner, the paleontologist throwing this idea around even implied that this could have been another path towards intelligent life aside from our own.

I can't yet find any evidence to back this claim up, but if so it would be a fascinating discovery; the question of why we are the only species from the only pathway that led to such intelligent life is a huge one. If intelligent life only happened once, it suggests that being intelligent was not an important adaptation for survival, merely a grain of dirt on the tree, there only to be brushed off like a flea. I like that idea.

Anyway, becoming multi-cellular is a pretty huge step and it would be unjust to cram it all into this single post so I'll do more on that next time when we finally reach the more recognizable branch on the tree:

Animalia.

_{In case you still aren't familiar}

Thanks for reading!

All images CC0 Licensed

References: _{Ediacaran life | Mark McMenamin | The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution | http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0002098 | Apoikozoa | Estimating the timing of early eukaryotic diversification with multigene molecular clocks}