The organisms of Earth lived in a highly competitive environment, with predators adding to the danger that each cell faced. The surviving cells needed to be able to allocate resources to internal processes, to keep the cell functioning, as well as external processes, to avoid being eaten by another cell. This requirement was difficult to fulfill for single celled organisms.
If you only had enough time to do one chore but multiple chores needed to get done, the effectiveness of each chore will likely decrease if you don’t have more time to spend. Single celled organisms faced this same dilemma.
Evolution is based on working with what you have rather than aiming for an ideal. The cells that were now splitting their resources between internal and external processes couldn’t just decide to develop a way to gain more resources.
Instead, smaller cells that were very good at performing specific processes found their way into a larger host cell. The host cell allowed the smaller cells to live within them, but only if the guest continued to do its job. The process of two organisms working together is called symbiosis.
Some cells adapted to perform photosynthesis very well so they gave the host cell a steady source of food; chloroplasts are a descendant of these. Some cells were good at breaking down food into more usable energy forms; mitochondria are modern examples of these cells. Others were able to build protein structures that aided the host cell in producing proteins of its own; the nucleolus is an example of this type.
Regardless of the function, the overall effect was the same: the guest cells began to allocate more and more of their resources towards performing their specific process. With those processes taken care of by the guest cells, the host could allocate more resources toward external processes to keep the host and its guests safe from predators.
Over time, the guest cells allocated all of their resources toward performing their task for the host and in exchange were given the protection and energy to continue surviving. These were the first communities of cells, all living within a larger cell. We call this community a eukaryote.
Eventually, an adaptation that combined two copies of RNA together to form a more protected structure would arise, we call this double stranded molecule DNA. Over time another adaptation would cover the DNA with an internal membrane to give greater protection. This was the formation of a cellular nucleus and it became our main identifier for eukaryotes.
Around 650 million years ago, a decrease in greenhouse gases lowered the temperature of the planet so much that the Earth froze into what we call Snowball Earth. Our atmosphere usually plays the role of a warmth blanket and without it, the overall temperature of the planet lowered to the point where glaciers formed all the way through the tropics, almost to the equator. Oceans froze over, this was a trying time for the organisms living underneath the ice. Couple these harsh conditions with the constant threat of predation and it becomes almost impossible for single celled organisms to survive.
Symbiosis to the rescue again! Individual eukaryotes found that grouping up gave a better chance for survival of the group and therefore each individual’s chances would improve. Sometimes the individual cells would continue their individual functions but give some resources to the group, we call this a colony.
The cells belonging to a colony could potentially survive on their own but were much more likely to thrive utilizing the cooperation of the colony. Many organisms still utilize the colonial advantage, allowing them to accomplish functions greater than the individual. You are probably aware of colonies of microbes, algae, fungi, and even animals like ants and bees.
A group of eukaryotes that work together may follow the trend of the smaller cells that make them up. Over time the guest cells specified in a single function so that the host cell could allocate resources to other functions. Similarly, over time a group of eukaryotes would specify in particular functions to aid the entire group. These cells would give up on some processes entirely and not even utilize the part of their DNA meant for other functions.
While individual eukaryotes were unable to survive outside of the group, this specialization process meant the group could perform much more complex tasks as a whole. This was the beginning of multicellular life: a community of eukaryotic cells working together to increase the overall chance of survival.
During our most trying times in the history of life, it was symbiosis that allowed us to keep moving forward. Working as a group gave our ancestors new possibilities to survive and thrive whether that is within a single cell or a single organism. This is a lesson that should not be forgotten: the group is always more capable and powerful than the individual.