Increasingly Earlier Life

Every creature inhabiting Earth, including newcomers like humans, represents an unbroken line to primitive carbon-based life present on Earth nearer and nearer to the planet's probable inception some 4.54 billion years ago.

Gnat encased in amber

"What we normally think of as 'life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorus."
— Stephen Hawking, from a 2008 lecture

Chains of carbon atoms are common to all living things.

Biogenic Substances

Biogenic substances contain carbon which is a recognized marker for living organisms. Such substances, in the form of constituents, or as secretions from plants or animals, embody biomolecules which are the building blocks of life.

coal & oil	geologic-age constituents that have undergone changes
chalk & limestone	geologic-age secretions (e.g., marine animal shells)
grass & wood	contemporary constituents
pearls, silk & ambergris	contemporary secretions

Improbable Life

That life-forming elements came from dying stars in the far reaches of the universe make life on Earth nearly incomprehensible, if not improbable.

I tell my students, with a feeling of pride that I hope they will share, that the carbon, nitrogen, and oxygen that make up ninety-nine per cent of our living substance were cooked in the deep interiors of earlier generations of dying stars. Gathered up from the ends of the universe, over billions of years, eventually they came to form, in part, the substance of our sun, its planets, and ourselves. Three billion years ago, life arose upon the earth. It is the only life in the solar system.
— George Wald, Harvard Biochemist ⁽¹⁾

olive-green zircon crystal
by Rob Lavinsky

Earliest Biogenic Substances

The earliest recorded biogenic substances on Earth gets incrementally pushed back as scientists find and analyze new artifacts.

Until recently, graphite detected in rock dated 3.8 billion years ago was the earliest known biogenic residue of life.

UCLA geochemists recently identified specks of biogenic carbon encased in undisturbed, crack-free zircon crystals from Western Australia.

This discovery pushes back the earliest biogenic residue some 300 millions years to 4.1 billion years ago. Thus it places the probable earliest life closer to the time when Earth was formed 4.54 billion years ago.

"Life on Earth may have started almost instantaneously. With the right ingredients, life seems to form very quickly."
— Mark Harrison, UCLA geochemist

REFERENCES

• 4.1 Billion Year Old Australian Zircon Contains Graphite of Biological Origin, Sci-News.com 20 October 2015.
• Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon, Elizabeth A. Bella, Patrick Boehnkea, T. Mark Harrison, and Wendy L. Maob, PNAS, 10.1073/pnas.1517557112, 14 October 2015.

⁽¹⁾ From speech given at an anti-war teach-in at the Massachusetts Institute of Technology, (4 Mar 1969) 'A Generation in Search of a Future', as edited by Ron Dorfman for Chicago Journalism Review, (May 1969)

It Goes On

Model of L-serine molecule

Moments after the Big Bang, the universe contained hydrogen, helium, and trace amounts of lithium. Once stars coalesced, carbon and oxygen were produced.

Carbon and oxygen are synthesized when helium burns inside red giant stars.

Carbon in the universe is traceable to stars

Carbon-based Life

Carbon is the essential ingredient of life-sustaining organic molecules: carbohydrates; proteins; nucleic acids; and lipids.

Carbon allows for bonds that form three-dimensional molecular structures likened to, and modeled by, rings and chains. Such complex molecular structures provide the building blocks of life.

“In three words I can sum up everything I've learned about life: it goes on.”
― Robert Frost

Earth's Carbon

Earth is largely comprised of oxygen and silicon with comparatively little carbon.

Earth exhibits an observable and measurable carbon cycle analogous to a bookkeeper's columnar ledger of debits and credits. Carbon is exchanged between these reservoirs:

• Biosphere - sum of all ecosystems, or life zone;
• Pedosphere - outermost layer of Earth comprised of soil and soil formation;
• Geosphere - solid parts of Earth including, but not limited to, the lithosphere;
• Hydrosphere - combined mass of water on and under the surface of Earth; and
• Atmosphere - layer of gases surrounding Earth retained by Earth's gravity.

Earth's carbon cycle is a model for the ongoing flows between each of the reservoirs. Some carbon flows are slow (e.g., on the order of hundreds of millions of years), while others are fast:

• Slow Carbon Cycle - Over the span of geologic time (100 - 200 million years), chemical reactions, tectonic events, and volcanic events occur that transfers carbon between rocks, soil, ocean, and atmosphere.
• Fast Carbon Cycle - Plants, and phytoplankton in the ocean, absorb carbon dioxide (CO₂) from the atmosphere into their cells via photosynthesis, then return CO₂ via cellular respiration and microbial decomposition.

Fast carbon cycle depicting exchange of carbon between reservoirs

“Originally, the atoms of carbon from which we’re made were floating in the air, part of a carbon dioxide molecule. The only way to recruit these carbon atoms for the molecules necessary to support life—the carbohydrates, amino acids, proteins, and lipids—is by means of photosynthesis. Using sunlight as a catalyst the green cells of plants combine carbon atoms taken from the air with water and elements drawn from the soil to form the simple organic compounds that stand at the base of every food chain. It is more than a figure of speech to say that plants create life out of thin air.”
― Michael Pollan, The Omnivore's Dilemma: A Natural History of Four Meals

Monitoring Earth's Carbon

NASA will launch the Orbiting Carbon Observatory-2 satellite in July 2014. It will record atmospheric CO₂ from space. The data will be used to map CO₂ sources and sinks and to provide insight into its variability over seasonal cycles.

You will die but the carbon will not; its career does not end with you. It will return to the soil, and there a plant may take it up again in time, sending it once more on a cycle of plant and animal life.
― Jacob Bronowski

REFERENCES

• Carbon Cycle, Wikipedia.
• Foundations of Carbon-Based Life Leave Little Room for Error, NC State University Press Release, 13 March 2013.
• Orbiting Carbon Observatory, NASA.
• The Carbon Cycle, Holli Riebeek, Earth Observatory, 16 June 2011.
• What is the Carbon Cycle? What is the science behind it?, United States Carbon Cycle Science Program.

Cosmic Newcomers

Humans are cosmic newcomers.

It is commonly held that our home planet coalesced from cosmic dust and gas. Carbon-based life is a relatively recent event. Life is a viral outbreak that occurred billions of years ago on Earth.

The profundity of carbon-based life is that chemical organisms replicate and grow in complexity.

Life, and more specifically human life, is a latecomer to the existential table.

Humans are a puff, a wink, or a dream.

Men come and go, cities rise and fall, whole civilizations appear and disappear-the earth remains, slightly modified. The earth remains, and the heartbreaking beauty where there are no hearts to break....I sometimes choose to think, no doubt perversely, that man is a dream, thought an illusion, and only rock is real. Rock and sun.
― Edward Abbey, Desert Solitaire: A Season in the Wilderness

Many geologists think our atmosphere and oceans arrived on the Earth about 4 billion years ago either from a series of volcanic eruptions or from comets colliding with earth that released water and gases.

Evidence of Life

How long before life emerged from the chemical soup of oceans and atmosphere? The first evidence of life, inferred from the chemical record (e.g., Carbon isotope C-12 to C-13 ratio), dates from 3.85 to 3.65 billion years ago. The first undisputed fossils appear in rocks about 3.2 billion years ago. Human footprints appear 1,500,000 years ago. The first humans fossils known to us are from 195,000 years ago.

Artifact Years Ago Atmosphere and Ocean 4,000,000,000 Life 3,850,000,000 Fossil 3,200,000,000 Human Footprint 1,500,000 Human Fossil 195,000

I stand for what I stand on.
― Edward Abbey

REFERENCES

• Carbon-based Life, Wikipedia.
• How did our planet form?, Kate Ravilious for The Guardian, 27 April 2008.
• Oldest Human Footprints With Modern Anatomy Found, John Roach for National Geographic News 26 February 2009.
• Oldest Human Fossils Identified, Hillary Mayell for National Geographic News, 16 February 2005.
• When Did Life on Earth Begin? Ask a Rock., David Tenenbaum for Astrobiology Magazine, 14 October 2002.