Final answer:
The earliest organisms on Earth were prokaryotic cells, evolving around 3.5 billion years ago in an anaerobic atmosphere. They survived harsh conditions and eventually gave rise to more complex forms, including colonial organisms as precursors to multicellularity, leading to the diversified explosion of life in the Cambrian period.
Step-by-step explanation:
Evolution of Major Life Forms
The first forms of organisms on Earth were likely prokaryotic cells, which originated about 3.5 billion years ago. These simple single-celled organisms were the only life forms until the emergence of eukaryotic cells around 2.1 billion years ago. During the time of prokaryotes, we saw the evolution of photosynthetic prokaryotes, such as cyanobacteria, which significantly contributed to the oxygenation of the atmosphere. Early Earth's atmosphere lacked molecular oxygen and was rich in volcanic activity and radiation, requiring these first life forms to be anaerobic and tolerant to extreme conditions.
The progression from prokaryotic cells to more complex organisms includes an evolutionary transition to colonial organisms, which are considered to be a step towards multicellularity. Algae of the genus Volvox are an example illustrating this transitional stage. The further evolutionary pathway led to the diversification of life forms, illustrated by events like the Cambrian Explosion around 610 million years ago, where a burst of evolution introduced a variety of complex organisms.