Single cell creatures arose about 3.5 billion years ago while the first multi-cellular creatures arrived 1.5 billion years ago according to current thinking. Single cell life has dominated the Earth like no other.
Creatures such as us on the other hand, have been on the Earth at most 200,000 years or so. As mighty as we are, our tiny, microscopic forebears, the single-cell organism conquered and owned the Earth long ago.
Long before the first mammal let alone the first hominid came into existence, colonies of bacteria formed that began to break the Earth's microbial monopoly.
Unlike true multi-cellular organisms, this bridge from the one to the many have identical cells functioning individually. Volvox is a modern example of this colonial arrangement.
There is some debate on exactly how multicellular organisms arose as well as the era when it occurred. The best confirmed fossils show up around 650 - 450 million years ago with reports indicating worms may have been around at least a billion to 1.2 billion years ago.
Not in dispute is that a multicellular organism is composed of different cell types that do different things within the organism. A crucial difference between multicellular creatures and a colony, where every cell is the same.
Colonies from a successful adaptation perspective, display a strength-in-numbers argument, where in this case by a having a cohesive group of identical single-cell organisms will tend to exclude other types of cells from feasting on whatever exists at the core of the colony. This allows the colony an exclusive right to mine whatever exists within it's sovereign territory, so to speak.
Let's bypass the life of colonies and go straight to the Sponges, an interesting pack of organisms whose existence can be easily related to successful adaptation.
Sponges or porifiera are sea-creatures that are considered to be filter-feeders. They gain an advantage over other organisms by forcing more of the environment through themselves. As the environment passes through them, they take what they can from the passing stream.
Hydras or hydrozoans are another set of interesting creatures who existence spans both colonies and organsims, providing ample opportunity to study how early metazoan evolution occurred.
For the purposes of this hypothesis however, let us now depart from first and early life. There have been many books and papers written about early evolution and many still could be written to explain how successful adaptation can be related to every evolutionary advance we know about.