The Archean Period, the era of the bacteria

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Article 18: If you were able to travel back, in order to visit the Earth during the Archean, you would likely not recognize it as the same planet we inhabit today.

The Archean eon of the Earth’s history spans about 1,5 billion or 1,500 million years, from 4 to 2,5 billion years ago.

Between 4 billion and 2,5 billion years ago, the continental shield rock begins to form. Approximately 70 percent of continental landmass forms during this time. Small island land-masses float in the molten seas. The Earth has acquired enough mass to hold an atmosphere, composed of methane, ammonia and other gases. Water from comets and hydrated minerals condenses in the atmosphere and falls as torrential rain, cooling the planet and filling the first oceans with liquid water.

Archean environment

If you were able to travel back in order to visit the Earth during the Archean, you would likely not recognize it as the same planet we inhabit today. The atmosphere at that time, that mix of methane, ammonia and other gases, would be toxic to most life on our planet today. It is during this time that the Earth’s crust cools enough, so that rocks and continental plates begin to form.

The earliest evidence for life on Earth are biogenic graphite, found in 3,7 billion-year-old meta-sedimentary rocks, altered by metamorphism, discovered in Western Greenland, and microbial mat fossils, found in 3,48 billion-year-old sandstone, discovered in Western Australia. So it is early in the Archean that life first appears on Earth. Exactly when or how it happens is unknown, but microfossils of this time indicate that life begins in the oceans, about 3,5 billion to 2,8 billion years ago. It is probable that these microscopic prokaryotes begin as chemo-autotrophs, anaerobic bacteria able to obtain carbon from carbon dioxide (CO2). By the end of the Archean, the ocean floor is covered in a living mat of bacterial life.

Our oldest fossils date to roughly 3,5 billion years ago and consist of bacterial microfossils. In fact, all life during the 1,5 billion years of the Archean is bacterial. The Archean coast is home to mounded colonies of photosynthetic bacteria, called stromatolites.


Stromatolites have been found as fossils in early Archean rocks of South Africa and Western Australia. Stromatolites increase in abundance throughout the Archean, but begin to decline during the next period, the Proterozoic. They are not common today, but they are still doing well in Shark Bay, Australia.

Stromatolites 2
The stromatolites forming today in the shallow waters of Shark Bay, Australia, are built by colonies of microbes. Credit: University of Wisconsin-Madison

The Last Universal Ancestor (LUA), also called the Last Universal Common Ancestor (LUCA), is the most recent organism from which all organisms now living on Earth have common descent. This LUA is estimated to have lived some 3,5 to 3,8 billion years ago.

Charles Darwin proposed the theory of universal common descent through an evolutionary process in his book On the Origin of Species in 1859, saying, “Therefore I should infer from analogy that probably all the organic beings, which have ever lived on this earth, have descended from some one primordial form, into which life was first breathed.”

The very last sentence is a restatement of the hypothesis: “There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one.”

Tree of life

A phylogenetic Tree of Life, showing the relationship between species whose genomes had been sequenced as of 2006. The very center represents the Last Universal Ancestor of all life on earth. The different colors represent the three domains of life: pink represents eukaryotes (animals, plants and fungi); blue represents bacteria; green represents archaea. Note the presence of Homo sapiens (humans) second from the rightmost edge of the pink segment.

The concept of the Last Common Ancestor between two species, the LCA, is described in Richard Dawkins’ book, The Ancestor’s Tale, in which he imagines a pilgrimage backwards in time, during which we humans travel back through our own evolutionary history. As we do so, we are joined at each successive stage by all the other species of organisms with which we share each respective common ancestor. Dawkins uses the word Concestor as an alternative to LCA.

Common ancestor

In The Ancestor’s Tale, following the human evolutionary tree backward, we first meet the Concestor that we share with the species that are our closest relatives, the chimpanzee and bonobo. Dawkins estimates this to have occurred between 5 and 7 million years ago. Another way of looking at this is to say that our (approximately) 250,000-Great-Grandparent was a creature from which all humans, chimpanzees and bonobos are directly descended. Further on in Dawkins’ imaginary journey (imaginary, in that the journey is going backwards in time), we meet the Concestor we share with the gorilla, our next nearest relative, then the orangutan, and so on, until we finally meet, here in the Archean, the Concestor of all living organisms, known as the Last Universal Ancestor.

“We are inescapably the result of a long heritage of learning, adaptation, mutation and evolution, the product of a history which predates our birth as a biological species and stretches back over many thousands of millennia… Going further back we share a common ancestry with our fellow-primates and going still further back we share a common ancestry with all other living creatures and plants, down to the simplest microbe,” to quote Fred Hoyle in Lifecloud: The Origin of Life in the Universe (1978), page 15.

What it Means to be 98% Chimpanzee? is the cheerful title of a book written by Jonathan M. Marks. Yes, our genetic kinship with one of these fellow-primates, the present chimpanzees, is a very intimate one: 98,4%. But what a difference that 2% can make! Not even a casual observer would mistake a chimp for a human.

On the other hand: neither the universe and our solar system nor that wonderful simplest microbe are there in order to crown us, humans, as their Rex quondam, Rexque futurus, their Once and Future King.

Mark Twain reporting from Paris:
“If the Eiffel Tower were now representing the World’s age, the skin of paint on the pinnacle-knob at its summit would represent man’s share of that age and anybody would perceive that the skin was what the tower was built for. I reckon they would. I dunno!”

Thanks to the University of California, Berkeley, Museum of Paleontology, to LiveScience, and to Richard Dawkins for ‘The Ancestor’s Tale’

ShantiShanti is a regular contributor to Osho News
All essays of this series can be found in: At Home in the Universe
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