Eukaryotes and Oxygen -

An Evolutionary Connection

Published in The EarthSphere Blog. Feature Image: Evolution’s Journey (by ArcheanArt; Background modified from NASA public domain release)

Prologue

Previously in the Forgotten Origins series, we explored the rise of eukaryotic life in the aftermath of the Great Oxidation Event.

The Magic of Oxygen

Without oxygen, most life would cease. The element oxygen traces its history back some 13.3 billion years to a period a mere 500 million years after the universe came into existence. Astronomers peering into the past detected light from a distant galaxy, and in this ancient light was the radiation signature emitted by glowing oxygen atoms. The light in question arrived on Earth 13.28 billion years after being emitted, making it the oldest record of oxygen in our universe.

Oxygen atoms form deep in the interior of stars, where the pressures and extreme temperatures of nuclear fusion mold them. When these stars explode, they spew their oxygen across vast reaches of interstellar space. This oxygen eventually collapses into gravity wells along with other cosmic debris, forming planets. So the reality is, elemental oxygen existed on Earth from its inception. But when we speak of oxygen and life, we are not talking about elemental oxygen; we are talking about molecular oxygen (O2).

A single oxygen atom has limited relevance for cellular metabolism, but put two of them together, and the resulting molecule (O2) becomes the grand enabler of modern life. Anaerobic cellular metabolism uses inorganic molecules like ammonia, methane, or hydrogen sulfide and combines them with elemental oxygen to produce energy. But the process is inefficient. Molecular oxygen (O2) provides a significant efficiency boost to cellular metabolic processes.

Using molecular oxygen (O2), aerobic metabolism is eighteen times more efficient than anaerobic metabolism in producing ATP (Adenosine Triphosphate – the cell’s energy currency). The introduction of free oxygen (O2) into Earth’s atmosphere and oceans provided evolution with a tool for producing more complex lifeforms.

A Slow Process

The term oxygenated atmosphere only conveys the message that some appreciable amount of free oxygen was present; it does not specify what constitutes an “appreciable amount.” Oxygen levels at the end of the Great Oxidation Event were very low compared to modern-day levels. According to some researchers, oxygen did not reach its modern-day levels until rather late in the evolutionary process, about 450 million years ago.

Given this late arrival of a fully oxygenated Earth, eukaryotes must have muddled around for a billion years developing their survival skills before life as we know it arose. Even today, we know some eukaryotes thrive in low oxygen niches.

Banded iron formations, geological indicators of anoxic conditions in the oceans, tell a story of periodic regressions where free oxygen became scarce. Banded iron formations appear in the geological record about 800 million years before the present (BP); suddenly appearing after a hiatus of more than a billion years. The geological era was the NeoProterozic, and ample evidence supports widespread anoxic conditions in Earth’s oceans.

But the presence of these Neoproterozoic banded iron formations does not directly imply a world without oxygen. Clearly, portions of Earth’s oceans turned anoxic, but this does not mean total anoxia throughout all of the planet’s oceans. Environmental factors related to plate tectonics and volcanism may have regionally depleted the oxygen in large bodies of water without affecting other areas.

Researchers recently identified banded iron formations in Western China dated at 527 million years BP. These rocks imply anoxic condition well after the Cambrian Explosion when all the branches of modern life established themselves. Oxygen was and still is extremely important to life as we know it, but the process of fully oxygenating our atmosphere was a slow, plodding affair.

Help Needed

The introduction of significant free oxygen into Earth’s biosphere was unquestionably a seminal event in life’s evolutionary journey. But the billion-year gap between the eukaryotes’ first appearance and widespread, complex, multicellular life is an enigma. Why didn’t evolution progress more rapidly once efficient, oxygen-based cellular respiration became available?

The geological record remains foggy on this account, but several ideas deserve some speculative thought. Perhaps evolution is simply an exponential process, requiring a long build-up to critical mass. Homo sapiens emerged some 300,000 years ago and spent the next 295,000 years building up to the earliest human civilizations. Building momentum takes time. On the other hand, we have the Cambrian Explosion, where all modern biological lineages appeared in the geological blink of an eye. Then the rate of core biological diversification slowed in an un-exponential way.

But there are other possibilities. Maybe free oxygen was not enough, and more help was needed. Efficient cellular metabolism provided a huge advantage for eukaryote development and evolution, but we know from modern-day observations that other factors like nutrient availability sometimes limit species development. Is it possible that nutrient scarcity put a damper on evolution’s joy ride?

We don’t know the answer. What we do know is something changed about a billion years after the first eukaryotes appeared. An odd cold spell descended upon our planet during the middle of the Neoproterozoic. The period from 720 million years BP to 635 million years BP is known as the Cryogenian, aka Snowball Earth. At the end of the thaw, soft-bodied multicellular life forms had gained a foothold in Earth’s oceans. Something had changed.