It is extraordinary how increasingly complex the tree of life appears the more you actually look at it. I never imagined at first I would come across things such as 'unikonts' and 'Opisthokonta' and 'Apoikozoa', and I had no idea I would discover so many unknown, unranked grey areas where plants, animals and fungi all seem to blur with things that are not quite animals, plants nor fungi. Check out the progress so far if you haven't already:
So last time we looked at Ediacarans and the start of complex life. Nobody is exactly sure but the movement from single celled life to multi-celled life may have come from competition on floorspace with lots of organisms clumping together in a blanket across nutrient rich floors before symbiotically working together.
Evidence of this may come in the form of Ediacaran life found in fossils as flat, complex organisms, but the debate continues to this day.
The main quandary in this idea is why: Why bother becoming multicellular when life being single is clearly beneficial, having had a lifetime 2 billion years over multicellular life. There is nothing in the birth of multicellular life that suggests ditching the group and doing their own thing is a bad idea. In fact, the very idea of abusing the system and benefiting more as a singular item seems biologically tempting indeed.
According to NASA:
...a group of microbes that secretes useful molecules that all members of the group can benefit from can grow faster than groups that do not. But within that group, freeloaders that do not expend resources or energy to secrete these molecules grow fastest of all.
Cancer is a classic example of single cells thriving in a way that f***s it up for everybody else.
But this behaviour is even seen in complex life, such as cuttlefish. Earlier on Steemit I wrote about how a certain percentage of cuttlefish actually steal mates from bigger cuttlefish with clever deception, but if caught they can be in a world of trouble. In this case, the risk-reward ratio must have balanced out so that only a certain number of thefts would be tolerable before things started breaking down in their society. The same could arguably be applied to Humanity.
In the case of single celled life, it seems likely that they actually took advantage of team work and lone-wolfing the situation. There are examples of bacteria today that work together to create a thriving society, but then collectively cheat the system by not putting in the effort once they feel it's not necessary for themselves. Once they all decide that, the colony just... dies.
You may notice the fatal flaw in these lifeform's methodology. I'll give you a second to take a guess...
Got it yet?
Last chance....
The problem lies in their uniformity. If everybody is doing the same job, the responsibility is diluted among them and each individual starts backing out when they feel it's safe to be lazy. Think of it as the earliest form of Socialism.
What appears to happen, through mechanisms unknown as of yet, is that different tasks started to be handed out to different members of the colony, making each of them mutually dependent on one another. This way, those who slack off instantly get noticed and everybody including themselves start to pay the consequences. The group may only be stable if everybody is pulling their wait in their individual assigned tasks.
The added benefit of this is that there is a forced forward momentum in complexity, with a minimal risk of reversion. This is described as a ratcheting mechanism, which allows momentum of machinery in one direction, but not the other
This process gets much more complicated than it sounds, involving G and I cells, and strange equations that look like this: 
Needless to say, evolution found a potential mechanism that for the most part punishes solo activity and promotes teamwork, and this may have been the very first example of Symbiosis in nature - long before the often cited mitochondrial-cell symbiosis with which we all live with today.
As complexity increased, more jobs would have been divvied up through the generations until you basically end up with modern life; vast biological factories of cells divvying up odd jobs to keep the machines working.
So where to next?
Now we've crossed the main hurdle after a mere 2 months since I started this series, we can continue down the tree to more substantial, visible organisms: Animals.
We first pass the most basic forms of this life that seem somewhat reminiscent of their bacteria blanket ancestors, including sponges (7,500 known species), Placozoa (ONE lonely, lonely species), Comb jellies (167 known species), and Jellyfish & Corals (15,000 known species).
But this is where we see a significant feature:
Bilateria
Bilateria is a feature in animals that show a clear bilateral symmetry within their bodies. Everything from this point will share this feature that all previous organisms lacked. But why? Why is bilateria so significant that it consists of 1.35 million species?
Find out next time!
References: NASA | Stabilizing multicellularity through ratcheting | Ratcheting the evolution of multicellularity | Bilateria
Image Sources: Virginia Tech | all others CC0 Licensed