The formation of the Great Barrier Reef began millions of years ago. Volcanic activity created what is known as the Coral Sea Basin and a series of raised islands surrounding the basin. As the sea level changed over time so did life living in the basin and on the islands. Over time coral reefs grew, died, and were buried under sediment from volcanic eruptions. This layer of rich sediment built up over time forming shelves. The earliest individual coral located on in the raised area now known as the Great Barrier Reef is thought to have formed a half-million years ago. Around 600,000 years ago complete corals began forming in the area and the origin of oldest of those still living today dates back 20,000 years. Sea levels in the area continued to rise until approximately 6,000 years ago. In the past, the water level evened out creating the ideal conditions for the formation of a fusion of coral and other plant and animal life. The shallow grounds created by years of volcanic activity and sedimentary build up combined with the warm waters and high levels of sunlight provide the perfect environment for the Great Barrier Reef to form and thrive.

What transition would best replace the bold words in the paragraph?