Final answer:
Over 3 billion years of evolution were required to perfect the cell, with the transition from simple uni- or multi-celled life to humans occurring over several hundred million years. Complex multicellularity evolved at least ten times across various lineages, setting a foundation for diverse life forms, including humans.
Step-by-step explanation:
It took over 3 billion years of evolution to perfect the cell and only several hundred million years to go from simple uni- or multi-celled life to humans. Life on Earth began about 3.5 to 4 billion years ago with single-celled organisms similar to bacteria. With evolution over this extensive period, complex life forms such as multicellular organisms developed. The emergence of multicellularity marks a significant milestone, which occurred around 610 million years ago. This evolutionary transition set the foundation for a rapid diversification of life, particularly evident during the Cambrian Explosion.
The step from nucleated cells to complex multicellular life, including humans, involved the innovation of sexual reproduction, which seems to have spurred evolutionary diversification. Not all organisms evolve at the same rate; for instance, humans underwent millions of years of evolution from a now extinct mammal ancestor as opposed to bacteria, which can evolve over a thousand generations in a matter of days or weeks. All life forms, with the exception of viruses, display cellular structure. This ranges from simple single-celled organisms to complex multicellular entities. These larger multicellular entities, however, begin their life as a single cell, via the union of two gametes, highlighting the continuity in the evolution of life forms.