Evolution of life on Earth began about 3.5 billion years ago but it has not been a constant or continuous process.
During the middle years of Earth’s history (1.8 billion to 800 million years ago), evolution stagnated. Life remained as little more than a layer of slime for a billion years. This period has become known as the “boring billion” years.
So what was going on? A research team led by geologists at the University of Tasmania has developed new mineral technology to track the trace metal content of the ocean and oxygen content of the atmosphere over the past 3.5 billion years. This has never been achieved before.
Why is it important? Evolution of life in the oceans is strongly influenced by trace metals, as many metals (such as copper, zinc, cobalt and selenium) are taken up by marine species and are critical for life and evolutionary change.
Studying the ocean floor
Our UTAS research team – of which I was a part – with help from many other international geologists, have been collecting seafloor sediments from all around the world over the past six years.
We found pyrite (iron sulfide) in each sample and analysed for 22 different trace metals with a cutting edge laser system at UTAS, and built a unique database of more than 3,000 pyrite laser analyses to track changes in ocean chemistry spanning a 3.5-billion-year period through time.
Some exciting and totally unexpected outcomes emerged from this ocean tracking technology. The most significant outcome relates to how trace metals in the oceans have influenced the evolution of life.
Back in the early part of Earth’s history, from 3.5 billion to 1.8 billion years ago, single celled life evolved slowly but progressively, related to an abundance of available trace metals in the oceans. But during the “boring billion”, from 1,800 million to 800 million years ago, evolution slowed. This has been a puzzle to scientists.
Ocean life nearly collapsed
Our research, published in the Earth and Planetary Science Letters, suggests that the reason for the slow down is that the trace metal content of the oceans declined. This resulted in a depletion of critical trace metal nutrients to the point that oxygen content dropped and life in the oceans was in great danger of total collapse.
But rather than causing a mass extinction, marine life and evolutionary change was put on hold for a billion years.
Following the boring billion, our research shows that the trace metal content in the oceans rose steeply in a series of steps over a 200-million-year period, from 750 million to 550 million years ago.
This was accompanied by a steep rise in oxygen in the atmosphere (known as a Great Oxidation Event, see below) that led to the Cambrian explosion of life and progressive evolution to the present time.
The essential trace elements
Bio-essential trace elements are critical to life and evolution. These include cobalt, selenium, copper, zinc, molybdenum, vanadium and cadmium. Certain species need these trace elements to survive.
The elements are linked into the chemical structure of the cells and become a natural nutrient for survival. Cobalt is a central atom in the structure of vitamin B12, whereas zinc is essential for growth in many species.
The UTAS research team showed that at certain periods of earth history these trace elements were in short supply (such as the boring billion period) leading to evolutionary decline, whereas in other periods the bio-essential elements were in great abundance, causing rapid evolutionary change.
The Cambrian Explosion
This was accompanied by major diversification of other organisms. Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organised into colonies.
Over the following 70 million or 80 million years, the rate of evolution accelerated by an order of magnitude and the diversity of life began to resemble that of today.
The Cambrian explosion has generated intense scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s. In 1859 Charles Darwin discussed it as one of the main objections that could be made against his theory of evolution by natural selection.
The long-running puzzlement about the seemingly abrupt appearance of the Cambrian fauna 540 million years ago centres on three key questions:
- was there really a mass diversification of complex organisms over a relatively short period of time during the early Cambrian, and are we lacking evidence of what really happened?
- what might have driven such rapid change – was it all due to rising oxygen?
- implications about the origin and evolution of animals?
This latest research by the UTAS team demonstrates, for the first time, a rapid increase in bio-essential trace elements in the ocean starting 660 million years ago. So was this the cause of the Cambrian explosion of life?
Great Oxidation Events (GOEs)
GOEs are large increases in oxygen in the Earth’s atmosphere and there have been two in Earth’s history – one at 2.4 billion to 2.5 billion years ago and one at around 700 million to 550 million years ago corresponding with the Cambrian Explosion.
There are several schools of thought about GOEs' origin. The most favoured theory is that the GOEs are produced by a dramatic increase in ancient marine organisms (cyanobacteria) that released oxygen as a by-product of photosynthesis.
But which came first? Did the increase in oxygen speed up evolution of life or did an increase in life result in a rapid rise in atmosphere oxygen?
Either way, the oxygen did eventually accumulate in the atmosphere, providing a new opportunity for biological diversification as well as tremendous changes in the nature of chemical interactions between the atmosphere, rocks, oceans and living organisms.
The research team at UTAS, using a novel approach to the problem, demonstrated major changes in trace element concentrations in the ocean at both GOEs, which may be the answer to the rapid expansion of life.
This is the start of a new journey for the Tasmanian research team and we will be doing much more with this technology.
But it’s already becoming clear that there have been many fluctuations in trace metal levels over the millennia and these may help us understand a host of events including the emergence of life, fish, plants and dinosaurs, mass extinctions, and the development of seafloor gold and other ore deposits.