EARTH'S COMPLEXITY REVOLUTION

Today, nearly four billion years after life first appeared on Earth, the planet hosts an abundance of complex life. Life and the global environment have co-evolved together such that neither would exist in its present form without the other. The varied and complex life on the planet today both maintains, and is supported by, fertile land and oceans and an oxygen-rich atmosphere. But when did the Earth system as we know it come into being?

There have been three critical ‘revolutions’ in Earth history that have brought the planet to this point: (1) The ‘Inception’ starting with the origin of life ~3.7 billion years ago and the establishment of recycling ecosystems fuelled by anoxygenic photosynthesis; (2) The ‘Oxygen revolution’, started by the origin of oxygenic photosynthesis ~3.0 billion years ago and culminating in the Great Oxidation ~2.3 billion years ago; and (3) The ‘Complexity revolution’, starting with the origin of eukaryotes and culminating in the rise of oxygen to modern levels.

I will concentrate on the nature and timing of the Complexity revolution. Many recent studies have focused on the origins of animal life in the Neoproterozoic Era and a putative rise in oxygen at this time. However, geochemical data indicates that the deep oceans were not fully oxygenated until midway through the Paleozoic Era. I will argue that the rise of plants and complex ecosystems on the land surface first increased atmospheric oxygen to modern levels. Subsequently, both land and ocean ecosystems have played key roles in feedback mechanisms that stabilise atmospheric oxygen, carbon dioxide and climate. I will explore the ensuing relationship between biological complexity, diversity and Earth system stability.