The cause of animal evolution halts for two billion years

Scientists around the world have recreated the chemical structure changes under ancient oceans over a long geological period, between 0.5 and 2.5 billion years ago. They found that the lack of oxygen and heavy molybdenum deep in the ancient ocean could have caused evolution on Earth to evolve over a period of 2 billion years.

The researchers obtained this result by searching for traces of molybdenum in black shale rocks, a sedimentary rock rich in organic matter and often appearing deep in the ocean floor. Molybdenum is an important micronutrient for life, acting as an agent of oxygen in the air and in the atmosphere.

Following the initial rise of oxygen in the atmosphere 2.4 billion years ago, oxygen was transferred to the ocean surface to nourish oxygen-requiring microorganisms. However, according to Timothy Lyons, a professor of biochemistry at the University of California School of Earth Sciences, Riverside - one of the authors of the work - the diversity of these unicellular life forms is still low and Multicellular ancestors, animals, did not appear until 600 million years ago.

Suspecting that the lack of oxygen and molybdenum could explain the delay in this evolution, Lyon and his colleagues measured molybdenum's excess in ancient marine sediments to assess the amount of dissolved metal in seawater formed. So sediment samples. Scientists have discovered direct evidence, notably that the ocean in the past lacks of molybdenum is related to this high content in today's oxygen-rich waters.

Molybdenum.(Photo: Monica Graeme)

Molybdenum is of particular interest because some bacteria use it to convert nitrogen from an atmosphere in the atmosphere to a useful form of life - a process called "nitrogen fixation" . Bacteria cannot fix nitrogen effectively without molybdenum. And if bacteria can't fix nitrogen fast, eukaryotes - a creature that includes plants, thick skin and humans - will be in trouble because eukaryotes cannot fix nitrogen itself.

'Molybdenum deficiency can inhibit the development of complex life, such as animals, for nearly 2 billion years of Earth history. The amount of molybdenum in the ocean may play a major role in the development of early life. Similar to the case of iron today, molybdenum is thought to be a micronutrient that determines life to regulate the bio-nitrogen cycle under the ocean. '

'At the same time, early ocean molybdenum levels were related to the global impact of oxygen-deficient seawater and meant that atmospheric oxygen was still low. Understanding the amount of oxygen in the early ocean is important for many reasons, including accurate knowledge of how and when a significant amount of oxygen begins to accumulate in the atmosphere. The steps in the oxidation process are factors that make up the first animal species nearly 600 million years ago - only about one tenth or equivalent in Earth's history. ' Lyons said.

Oxidation of the Earth

Life wants to form, exist and proliferate on Earth, the threshold of oxygen requires 1-10% of the oxygen in the current atmosphere. Previous studies have shown that oxidation on earth occurs in two main steps:

* The first step took place 2.4 billion years ago when the ocean turned to a stage where only the surface was oxidized thanks to photosynthetic bacteria while the ocean floor was nearly oxygen free.

* The second step, about 600 million years ago, marks the period when the entire ocean is oxidized through a process that has not yet been discovered.

Clinton Scott, a doctoral student working at the Lyon laboratory and the first author of this work, said 'We want to know the state of the ocean between these two periods. By tracing molybdenum in shale rich in organic matter, we found that the deep ocean still lacks oxygen and molybdenum after the first step. This condition may have a negative effect on the evolution of early eukaryotes, our single-celled ancestors. Molybdenum data also tells us that deep ocean has been completely oxidized about 550 million years ago. '

According to Scott, the time of oxidation steps shows important events in the history of the Earth involved. Scientists have long calculated that the evolution of the first animals in some way relates to the assumption of the Snow Globe, that the Earth is covered from one extreme to the other by a layer of ice. thick for millions of years at a time."The second step of oxidation took place shortly after the last phase of the ice age that ended about 600 million years ago. So a question arises: Will this global freeze play a role. Is it important to increase the amount of oxygen and does this trigger the evolution of animals? '

This work was funded by the US National Science Foundation, the Department of Earth Sciences and NASA Space Biology, which appeared in the March 27 edition of Nature.

Learn about the role of molybdenum

Molybdenum, a metal that is abundant in the ocean today but less than in the past, is a perfect ancient chemical marker for two reasons. First, the original molybdenum source for the ocean is the oxidative wear layer of the continental crust, requiring oxygen in the atmosphere. Second, molybdenum is lost from marine sediments, which lack oxygen and excess sulfur. So the molybdenum fertility in ancient shales rich in organic matter requires atmospheric oxygen with a high sulfur content and requires little or no oxygen deep in the ocean. This combination is rare today but probably popular in the past, when oxygen is not as abundant as it is today.

When oxygen exists in the air, the amount of molybdenum dissolved in seawater is determined by the size of the sediments containing hydrogen sulfide and water in the seabed (the lowest, coldest and most distinctive layer of water positive). Where the sulfuric environment spreads, the content of the molybdenum remains small, increasing as the sulfuric environment declines. The amount of molybdenum in seawater is reflected in the size of molybdenum in shale deposits in the ocean floor.

The research team under the University of California, Riversides estimated the magnitude of the seas, the size of the sulfur-containing bottom water and the sediments, based on the molybdenum concentration in the slabs. Ancient black lightning. They do this by dissolving the sample into an acid compound and analyzing dissolved rock to obtain a dense result using a mass spectrometer. This amount of metal in shale detects the oxygen state of the ocean and early atmospheres and shows the variable content of this essential component of life. The lack of molybdenum can delay the development of eukaryotes, including the first animals - our earliest multicellular relatives.