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The Rise of Complex Life

Published in The EarthSphere Blog. Cover Image: Cell in Action ( ArcheanArt: modified from Wikimedia Public Domain Image)


Previously in the Forgotten Origins series, we explored Earth’s passage from a world with no free oxygen to a planet with ample oxygen for life to exploit along its evolutionary journey.


Often people look back on their lives and identify turning points where events transpire to significantly alter the future. Perhaps an inspirational mentor or new job opportunity launched a successful life-long career. Maybe a chance meeting resulted in new relationships and unexpected lifestyle changes. Think of the planet’s Great Oxidation Event 2.4 billion years ago as one of the most significant turning points in Earth’s history.

The first two billion years of life on Earth was primarily occupied with single-celled organisms drifting in Earth’s salty chemical-laden oceans, trying to maintain their tenuous foothold in the primordial biosphere. Bacteria and archaea were the pinnacles of life—tiny prokaryotes with their precious DNA floating inside a cell membrane. They ceaselessly extracted energy from their environment, working to fulfill their singular destiny: produce more offspring cells, and ensure the future of the species.

Capturing energy is hard work, and before free oxygen, all of life depended on relatively inefficient metabolic pathways. Modern manufacturing closely tracks productivity, measuring how much resource is needed to produce a set quantity of end products. Since traditional manufacturing relies heavily on human input, we often think of productivity in terms of output products divided by people employed. Fewer people producing more products is an improvement in productivity. Henry Ford’s first automobile assembly line realized a hundred people on an assembly line produced more affordable cars than a hundred people each building their own cars from start to finish. He improved productivity and efficiency by using fewer resources to make more products. Early life was waiting for the opportunity to produce more energy with less work, and free oxygen was the turning point.

The First Eukaryotes

Today, virtually all visible life consists of multi-cellular organisms, which in turn are composed of eukaryotic cells. Cellular metabolism in these eukaryotes depends on the availability of free oxygen. Perhaps the arrival of free oxygen 2.4 billion years ago was the evolutionary stimulus needed for eukaryotes to make their grand entrance onto the stage of life. But this connection is more theory than fact. As expected, disagreement exists in the scientific community about exactly when eukaryotes developed.

The National Center for Biotechnology Information places the origin of the eukaryotes at 2.7 billion years ago. Alternative dates of 2.0 billion years and 1.5 billion years were proposed by researchers in articles from Scientific American and Max-Planck-Gesellshaft. One of the issues for establishing when eukaryotes first appeared on Earth is the type of evidence used. For biological investigations, fossils and biomarkers are the two most used indicators.

Fossils are firmer evidence than biomarkers, and the oldest fossil evidence of eukaryotes dates to between 1.7 and 1.5 billion years ago. Biomarkers push the date of origin back to 2.7 billion years before present (BP), but controversy remains about whether the detected biomarkers are simply due to sampling contamination. This range of dates spans a period before and after the Great Oxidation Event.

Another approach is to investigate and date phylogenies, timelines based on heritable traits, which determine a species’ history of evolution. DNA, morphology, protein structure, and more are used in phylogenetics. Phylogenetic trees for eukaryotes point to their emergence about 2.0 billion years BP.

Circumstantial Evidence

Perhaps a more important observation lies in the known dependence of eukaryotic life on free oxygen. It becomes difficult to see how these organisms could flourish before adequate oxygenation of our atmosphere and oceans. Possibly, pockets of eukaryotic life developed before 2.4 billion years BP as biological processes fleetingly gained the upper hand over geology in the biosphere. The biomarker evidence for his is tenuous, but nonetheless, it’s worth considering.

Regardless of the exact date of inception, circumstantial evidence indicates that the rise of eukaryotic life as a dominant evolutionary branch in the biosphere occurred after significant oxygenation of the atmosphere and oceans. The Great Oxidation Event was a major turning point, a pivotal occurrence probably responsible for me pondering and writing about it and for you reading my musings.

After a slow two billion year start, life began to gain momentum when oxygen appeared. Life is at its best when exploiting its local environment and finding the most efficient way to extract energy and propagate itself into the future. Species that are successful at this game rise to dominate their ecosystems, at least until another interloper finds an even more efficient evolutionary trick. The advantage of oxygen-based cellular metabolism was a big trick to learn.

(Next: What lies behind the magic of oxygen?)



The Cell: A Molecular Approach. 2nd edition (Source: NCBI) 

When did eukaryotic cells (cells with nuclei and other internal organelles) first evolve? What do we know about how they evolved from earlier life-forms? (Source: Scientific American) 

Eukaryotes: A new timetable of evolution (Source: Max-Planck-Gesellshaft) 

What Are the First Eukaryotic Fossils? (By Marina Somma; Sciencing) 

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.