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Earth’s earliest life forms developed ways to survive the harmful effects of arsenic to cope with dramatic changes in their environment, a new study suggests.

The researchers found the complex life forms, called eukaryotes, stored arsenic inside special compartments within their cells, a strategy that helped neutralise the toxic poison.

Using advanced X-ray technology, the international team was able to detect and map arsenic within 2.1-billion-year-old fossils from the Francevillian Basin in Gabon.

The arsenic found in the fossils was not due to later contamination but part of a biological response to environmental stress, according to the team.

a Lobate fossil with imprint showing dispersed pyrite grains at the radial fabrics (RF) and coagulated pyrite towards the dome (D) (b). c Elongate fossil (E) in a matrix (M) with imprint surrounded by bacterial mat33 showing mainly coagulated pyrite crystals (d). e Tubular fossil (T) having an ovoid diameter filled with coagulated pyrite in a matrix (M) (f). g Pyritized abiotic concretion (C) found in the same location and containing massive pyrite (h). The diameter of the coin in (g) is 3.6 cm. Scanning electron microscope is used in back-scattered electron mode in b, d, f and h. It shows pyrite grains filling the specimens in a vertical transect — Nature Communications.

This is revealed by distinct patterns formed from the arsenic preservation process in the fossils when compared to structures left by non-living mineral structures; it is further evidence the fossils were once complex living organisms with more advanced cells, they argue.

Their study, published in Nature Communications, reshapes current understandings of how early life faced environmental challenges, highlighting the critical role adaptation played in the evolution of life.

“The ability to cope with arsenic was not something eukaryotes developed randomly,” said Dr Ernest Chi Fru, one of the paper’s co-authors and Reader at Cardiff University’s School of Earth and Environmental Sciences.

“It coincided with a period of significant environmental change, when oxygen levels in the Earth’s atmosphere first rose. This increase in oxygen also led to a rise in arsenate, a particularly toxic form of arsenic which competes with phosphate, a vital nutrient for all life, making Earth’s oceans a dangerous place.”

a Oxidized As—arsenate or As5+ (red dots) is released into the ocean by the chemical and oxidative weathering of the continental crust. b During life, As enters the cell, and is detoxified first by reduction to As3+ (c) then extruded by cell membrane transporters or sequestered in intracellular compartments (d). e After death and burial, As is released from intracellular bodies into a localized reduced environment rich in iron (purple dots) where sulfate (green dots) is reduced to sulfides (yellow dots). f Pyrite is formed by microbial sulfate reduction with As acting as a catalyzer of pyrite nucleation. g Arsenic is rapidly consumed in low environmental concentrations. h Pyrite growth led to As depletion away from the core — Nature Communications.

The study builds on the team’s previous work on the 2.1-billion-year-old Francevillian biota, which they argue appeared after a local underwater volcanic event brought a sudden surge of nutrients into a small, enclosed sea.

This nutrient boost helped these early life forms thrive locally, according to the team led by Université de Poitiers and Cardiff University.

Dr Chi Fru added: “We looked at the evolution of arsenic in the Francevillian basin’s seawater before and after the fossils. It was actually quite low in arsenic concentration at the time when these primitive eukaryotes evolved, leading us to think they should have lived there quite happily.

“However, the surprisingly high levels of arsenic stored in their bodies, revealed in our analysis, suggest that they were extremely sensitive to even low levels of arsenic in seawater.”

These organisms later became extinct when volcanic activity returned to the area, and oxygen levels in the seawater dropped, according to the team.

They say their disappearance suggests the ability of complex life to protect itself from toxic substances like arsenic, by safely storing it inside cells, may have evolved more than once in Earth’s history.

“All living things have ways to protect themselves from arsenic, which is toxic to life,” Dr Chi Fru said.

“In the ocean, tiny plankton near the surface — the same ones that make about half the oxygen in the air we breathe — are always working to get rid of arsenic from their bodies. They can’t avoid it because arsenic is naturally in the water, and their cells can’t easily tell the difference between arsenate and phosphate, a nutrient they actually need. This was true even in ancient times, just like it is today.

“We know these ancient organisms went extinct, so the way modern life handles arsenic didn’t come directly from them.”

The paper, ‘A battle against arsenic toxicity by Earth’s earliest complex life forms’, is published in Nature Communications. (open access)

Astrobiology