Species that were unable to adapt to the changing environment and competition from other life forms became extinct. Cells within colonies became increasingly specialized, resulting in true multicellular organisms.
However, the fossil record retains evidence of many of these older species. Current fossil and DNA evidence shows that all existing species can trace a continual ancestry back to the first primitive life forms. The advent of photosynthesis in very basic forms of plant life worldwide allowed the sun’s energy to be harvested to create conditions allowing for more complex life.
The resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of yet more complex cells called eukaryotes.