Final answer:
Around 400 million years ago, animals began to move onto land from the waters, with significant adaptations facilitating this transition. The Cambrian explosion marks a vital point in diversification, but the Ediacaran biota represents an earlier stage of complex life. The amphibian era peaked around 300 million years ago, making way for the rise of reptiles, birds, and mammals.
Step-by-step explanation:
The period when creatures are believed to have transitioned from the waters to become the animals we know today is generally placed around 400 million years ago. Significant events in the evolutionary history of life on Earth saw multicellular organisms emerge during the Ediacaran Period, approximately 635 to 543 million years ago. These organisms, known as the Ediacaran biota, were the first complex life forms. Following this, during the Cambrian period approximately 530 million years ago, there was a vast diversification of animal species, a phenomenon referred to as the Cambrian explosion.
Animals started venturing onto land about 400 million years ago, with structural changes that enabled them to adapt to terrestrial life. For instance, developments such as a resistant outer skin to prevent drying out, body cavities for breathing atmospheric oxygen, and modified limbs for movement were crucial adaptations for terrestrial existence. The amphibians had their heyday approximately 300 million years ago, leading to the eventual rise of reptiles and, subsequently, birds and mammals from reptilian ancestors.