May 7, 2018

The Story of Life on Earth (Part I)

Life on Earth
Illustration by Ian Haydon

“To see a World in a Grain of Sand
And a Heaven in a Wild Flower
Hold Infinity in the palm of your hand
And Eternity in an hour”

The beginning of William Blake’s Auguries of Innocence

Primordial water, the beginning of infinity

If life didn’t come from space, the origin of life likely occurred either in primordial ponds that resemble tidal pools, or in deep sea hydrothermal vents. More than 4 billion years ago, the young Earth (~ <500 million years “young”) had conditions that favored the generation of some of life’s basic building blocks: nucleic acids, amino acids, and lipids. [1] These macromolecules gave rise to prototype cells that would eventually lead to the earliest life forms. Debaucherous horizontal gene transfer (i.e. microbial sex) ran rampant among the earliest life forms.

The cradle of life in abyss, the birthplace of Archaea and Bacteria

The exact details about how the earliest life forms evolved into the Last Universal Common Ancestor (a single-celled, bacterium-sized organism) and the amount of time that took were unclear. The following is well understood by scientists:

The cradle of life where the Last Universal Common Ancestor (LUCA) evolved was no Eden: it was the deep sea vents, a magma-bleeding abyss where sunlight could not reach. If the LUCA or any life forms existed during the Late Heavy Bombardment (4.1-3.8 billion years ago), they were sheltered within the deep sea, untouched by the Bombardment’s planetwide meteorite rain of fire. [2] From this distant world, the LUCA gave rise to two earliest domains of life: Archaea and Bacteria.

The untold story of oxygen

Early archaea and bacteria thrived in an anaerobic world, as Earth’s atmosphere was once devoid of oxygen. A bit less than 2.5 billion years ago, the rise of the oxygen-farting cyanobacteria (i.e. blue green algae) set off cascades that would eventually lead to our modern atmosphere that is oxygen-rich (21% of air). Complex life forms like us owe the atmospheric oxygen they depend on to these lowly algae. (Note: plants’ photosynthesizing ability has a cyanobacterial origin.) As the Gaia Hypothesis has suggested, organisms are shaped by the environment, which in turn shapes the environment, sometimes quite profoundly.

The symbiotic origin of Eukarya

2 billions years ago, two microbes — one archaeon and one bacterium cruising in the deep sea vents (still!) — had an interspecies “sex” that would change the world. One devoured the other. Two became one, giving rise to the progenitor of the last domain of life — Eukarya (all fungi, plants, and animals are eukaryotic organisms). Over many lifetimes, the swallowed bacterium evolved into the mighty mitochondria, power-generating organelles all eukaryotic life forms depend on. (Fun fact: like most animals’, human mitochondria have exclusively maternal genome, i.e. only from the mother’s egg. The sperm mitochondrial genome is quickly destroyed upon fertilization.) The thing about Eukarya is that they gained incredible energy efficiency through compartmentalization (and later also through specialization).

The quantum leap of complexity that revolutionize evolution

For billions of years, life existed only in simple forms. Around 542 million years ago, a new era of biocomplexity known as the Cambrian Explosion dawned upon Earth: sea animals that belong to the realm of fantasy were evolved, and plants and fungi started creeping onto lands. While there were many factors contributing to this moment, the abundance of free oxygen and the emergence of eukaryotic compartmentalization and specialization played pivotal roles. The Cambrian Explosion lasted for 20-25 million years. The evolution game was revolutionized, forever.

This is our history. This is our story.

Everything we take for granted — our own cells, our own body, our own world — is brimming with wonder and mystery, waiting to be discovered. Science helps us view the world in refreshing light. This article is written to encourage people to see the spiritual, story-like aspects of biology and science communication.

FOOTNOTES:

[1] Primordial nucleic acids were/gave rise to self-replicating RNAs that have enzymatic activity. RNAs are dynamic but unstable, they have existed and served as information carriers long before DNAs. Lipids provides the building blocks of boundaries, separating what is in from what is out, a hallmark of life.

[2] http://www.nytimes.com/2016/07/26/science/last-universal-ancestor.html?_r=1

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