Life’s Rocky Start – PBS/NOVA (2016)

Geology and biological evolution of life influence each other tightly. The title of the documentary “Life’s Rocky Start” reflects this relationship superbly. The six stage transformation of our planet from black, gray, blue, red, white to green is a wonderfully concise way of outlining the geological and biological evolution. More than half of the minerals now incorporated into the upper crust of our planet were produced by living organisms. The movement of continental plates has played a fundamental role in the recycling of mineral resources by the biosphere.

Formation of continents and ocean surfaces were prerequisites for life’s origins. Evidence is accumulating that continental crust formation started quite rapidly and much earlier than our previous estimates. For instance rock formations in northeastern Canada could be as old as 4.3 billion years old. That is a time not long after the planet’s formation and hundreds of millions of years before what is currently accepted as evidence for the most ancient life yet found on Earth.

Throughout Earth’s 4.6-billion-year history most rock formation have been destroyed through a geological process called subduction. Earliest rocks have been swallowed back into the mantle. Nevertheless, some of the 4-billion-year-old crust still remain. Researchers found isotopic evidence of some basaltic crust more than 4.3 billion years old mixed into some of the 2.7-billion-year-old rocks of the Superior Province in Canada. This is pretty impressive which means that at least one continental craton survived the subduction process. Geological time capsules known as zircons form the basis of evidence for earliest rock formations.

Zircons: Time Capsules from the Early Earth – Science Bulletins – AMNH from Nature Documentaries on Vimeo.

How plate techtonics started is another curious question. Granite formation is necessary for continent formation. Granite form by diffusion of water deep into the Earth’s crust. A distinguishing geochemical characteristic of Earth’s crust compared to other terrestrial bodies in the solar system is that Earth’s continental crust contains less magnesium. Decrease in magnesium content is correlated with granite formation. Based on magnesium levels researchers have calculated that plate tectonics could have started at around 3 billion years ago.

Now that the Earth is formed, the next question is how life get started? There is no direct fossil evidence but chemical signatures provide evidence for possibly the earliest life on planet Earth. Curious fiber-like structures that is at least 3.77 billion but could be 4.28 billion years old found in Quebec, could be harboring minerals in the form of microtubule fossils manufactured by microbes living in the early hydrothermal vents (see video below). If validated the start of life on Earth could be 700 million years earlier than previous evidence. A separate study from Greenland suggests that structures that appear to look like stromatolites at around same time points to an independent new source for origins of life.

In addition to oceanic settings for the origins of life there’s also a competing hypothesis in parallel with that of Darwin’s “warm little pond”. For instance fossils of land-based microorganisms preserved in 3.5-billion-year-old geyserites in an extinct volcano in the Dresser Formation in the Pilbara region of Western Australia suggest freshwater environments. Geyserites are only found in hot freshwater springs. The fossils include stromatolites and circular voids trapped in the rock by gas bubbles that look like they formed inside microbial substances. Freshwater ponds also provide a perfect environment for formation of temperature, pH and concentration gradients. When these gradients occur in areas where there is a wet-dry cycle chemical ingredients required for life can have the opportunity to evolve from geochemistry towards biology. Clay minerals provide such suitable templates.

Oldest rocks on Earth carry no sign of oxygen. Photosynthetic organisms evolved as early as 3,5 billion years ago. However, it was the evolution of the oxygenic photosynthesis at around 2.4 billion years ago that began to split water and release oxygen. The Great Oxidation Event was the beginning of the oxygen rich atmosphere that we experience today. Free oxygen in the atmosphere began reacting with phosphorus and iron on land surfaces. Phosphates and iron oxides became more readily available as nutrients when they leached into aquatic environments leading to more microbial photosynthesis.

At around 659 million years ago a quite drastic shift happened from bacterial dominated oceans to algae dominated oceans. This was drastic because even the smallest algae is about 50 times bigger than a bacteria. This could be the necessary transformation during the Ediacaran period that set the stage before the Cambrian explosion.

The documentary lays out the key developments in the green phase of our planet that started with the Ediacaran period. If you want to learn about geo-biological events all the way until the famous Cambrian explosion, you might want to check out the first part of a BBC documentary series called “First Life”.

 

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