Scientists uncover the source of an almost 2 billion year delay in animal evolution

Chemicals in ancient deep oceans provide clues to speed of evolution

A deficiency of oxygen and the heavy metal molybdenum in the ancient deep ocean may have delayed the evolution of animal life on Earth for nearly two billion years.

Dr Simon Poulton, Civil Engineering and Geosciences, Newcastle University, was part of an international team of biogeochemists who took part in the University of California-led study.

The study’s results are published in today’s edition of Nature (27th March).

‘For decades it was assumed that the ocean became oxygenated shortly after an initial rise in atmospheric oxygen about 2.4 billion years ago,’ said Dr Poulton. ‘This study provides independent confirmation that there was a major delay in the oxygenation of the ocean, and furthermore, it now appears that the availability of molybdenum may have played a crucial role in animal evolution.

‘At last, a coherent picture of the environmental conditions that led to the evolution of animal life is emerging.’

The researchers arrived at their conclusion after tracking molybdenum in black shales, a kind of sedimentary rock rich in organic matter found in the ocean. Molybdenum is a key micronutrient for the life-forms that control the production of oceanic and atmospheric oxygen.

Following the initial rise of oxygen in the Earth’s atmosphere 2.4 billion years ago, oxygen was transferred to the surface ocean to support oxygen-demanding micro-organisms. However, the diversity of these single-celled life forms remained low, and their multi-cellular ancestors (animals) did not appear until about 600 million years ago.

Suspecting that deficiencies in oxygen and molybdenum might explain this evolutionary lag, the team measured the abundance of molybdenum in ancient marine sediments over time to estimate how much of the metal had been dissolved in the seawater in which the sediments formed.

The researchers found significant, firsthand evidence for a molybdenum-depleted ocean compared to the high levels measured in today’s oxygen-rich seawater.

‘These molybdenum depletions may have retarded the development of complex life such as animals for almost two billion years of Earth’s history,’ said project leader Professor Timothy Lyons, at the University of California’s Department of Earth Sciences. ‘The amount of molybdenum in the ocean probably played a major role in the development of early life.