THE FIRST SPARK OF LIFE — 3.8 BILLION YEARS AGO

What kind of organisms were the first?

The most ancient common ancestors of life on Earth are likely heat-loving microorganisms

that find oxygen toxic and that thrive off gases like methane and hydrogen sulfide

(extremophiles). These are the organisms that would have been able to get started in

the environment that marked early Earth. Fossil remains of microscopic organisms about

3.5 billion years old have been found at sites in Australia and Canada. There is evidence

Life is found today in the strangest of places

The region of Earth that contains life extends from the bottom of the deepest ocean to a

few miles or kilometers into the atmosphere. There are several million known kinds—or

species—of living organisms, and scientists believe that there are far more species not

yet discovered.

We can see conditions similar to early Earth and the living organisms that would have

evolved there in real places on Earth’s surface today, such as:

In African gold mines, in shafts nearly 3 km underground, at temperatures as hot as •

120 °F, there are colonies of bacteria surviving without sunlight or oxygen, making

their own food from energy stored in chemicals. Some of these organisms divide

and reproduce as slowly as once every thousand years. Other species of bacteria

have been found deep in mine shafts and rocks all over the world, bacteria that love

heat (thermophiles) and make their food from chemical reactions involving iron,

manganese, methane, and sulfur.

How did life survive early meteorite bombardment?

The density of craters on the Moon allows scientists to estimate the rate of large impacts

on the Earth during the early history of the solar system. During the intense asteroid

bombardment that ended 3.8 billion years ago, roughly 20 impacts large enough to

vaporize the oceans occurred. Such impacts would be planet-sterilizing, at least for any life

in the oceans. Or so scientists initially thought. But based on new isotope data suggesting

liquid water existed as early as 4.4 billion years ago, scientists have revised their thinking.

Between the meteor strikes, the surface could have cooled enough to allow the oceans to

re-form. Life could have evolved and gone extinct (or into hiding) multiple times as well.

We know that life can survive in protected places—we see it today in caves and thousands

of meters below the surface of the oceans and even kilometers deep in solid rock!

How and where did life first evolve?

This question has been the subject of research and debate for decades. One of the first

experiments to simulate the beginning of life was done by Stanley Miller as a graduate

student at the University of Chicago in the early 1950s. Into a closed system of flasks,

Miller put methane, ammonia, hydrogen gas, and water vapor—all materials that would

have been major components of the early atmosphere. He warmed a soup of these

chemicals, circulated them through a region where they were subject to electric sparks

(simulating lightning), and cooled them and returned the products to the soup. Within a

few days, the soup was a brown slime that contained amino acids, the building blocks of

proteins, in about the same proportions as found in organic matter in a meteorite.

for chemicals created by living things in rocks

that are 3.8 billion years old. And recent isotopic

data in 4.4-billion-year-old zircons suggest that

life could have existed as far back as then. Many

scientists now believe that life appeared on

Earth as soon as conditions allowed.

For most of Earth’s history, life consisted of

single-celled organisms. And for the first almost

2 billion years of life on the planet, these

cells—Archaea or Bacteria— lacked a nucleus.

Amazingly, these oldest, simplest life forms still

exist today!

Harvard paleontologist Andy Knoll (in an interview in 2004):

“We don’t know how hard it is to go from the simplest bricks, if you will, in the

wall of life to something that is complicated, like a living bacterium. We know that

it happened, so it’s possible. We don’t really know whether it was unlikely and just

happened to work out on Earth, or whether it’s something that will happen again

and again in the universe.

My guess is it’s not too hard. That is, it’s fairly easy to make simple sugars, molecules

called bases which are at the heart of DNA, molecules called amino acids which are at

the heart of proteins. It’s fairly easy to make some of the fatty substances that make

the coverings of cells. Making all of those building blocks individually seems to be

pretty reasonable, pretty plausible.

The hard part, and the part that I think nobody has quite figured out yet, is how

you get them working together. How do you go from some warm, little pond on a

primordial Earth that has amino acids, sugars, fatty acids just sort of floating around

in the environment to something in which nucleic acids are actually directing proteins

to make the membranes of the cell?”

What’s the evidence?

A 4.4 billion-year-old zircon grain, from a rock in Australia, was found to contain oxygen

isotope ratios indicative of water on Earth’s surface. Prior to this discovery, the oldest

evidence of water was 3.85 billion years ago. Since life requires water, 4.4 billion years

represents a possible oldest date for life on Earth, but is unconfirmed by fossil evidence.

A 3.85 billion-year-old rock, from an island in Greenland was found to have higher-than-

normal amounts of carbon-12, an isotope of carbon that is used by microorganisms to

construct their own organic building blocks. Finding a lot of 12C trapped within layers of

Although scientists no

longer think the components

of his experiment were

an exact match to early

Earth’s environment, Miller’s

experiment showed scientists

that it was possible to create,

through natural processes, the

materials needed to build life.

The oldest microfossils: unbranched carbonaceous filaments (species: Primaevifilum amoenum) described as degraded cellular fossil prokaryotes. Samples (a–d) are from the 3.465-billion-yeas-old Apex chert of Western Australia; scale shown in (d). Samples (e–i) are from the 3.496-billion-year-old Dresser Formation of Western Australia; scale shown in (i). Sample (g) is an interpretive drawing of the specimen illustrated in (f). Schopf ©

Ancient stromatolite (microbial mat) fossils from the 3.496-billion-year-old Dresser Formation, Western Australia. Schoptf ©

In hydrothermal vents, cold seeps, and •

hydrocarbon seeps on the bottom of the

seafloor, bacteria capture energy from

hydrogen sulfide and methane gases to

make their food. They live at the bottom of a

diverse and exotic food chain.

In limestone caves, near volcanic sulfur-rich •

hot springs, bacteria capture energy from

hydrogen sulfide gas to make their food.

Sulfuric acid is one of the toxic byproducts,

and these bacteria live in highly acidic

mats attached to the cave walls.

rock is a strong hint that life

may have been present.

A 3.5-billion-year-old rock

from Western Australia

contained fossilized

microscopic cells.

Stanley Miller tried to re-create conditions similar to the early Earth in the laboratory. A much older Miller is shown here with a re-creation of the experiment. © A. Knoll

Early Archaea may be the ancestors of all life on Earth. This specimen of Methanosarcina thermophila is an example of a methanogen, a type of Archaea that gets its life-sustaining energy from methane gas and is capable of living in the harsh environment of the early Earth. © Stephen H. Zinder

Cyanobacteria. Organisms like this first appeared in the fossil record 3.5 billion years ago. They lived in colonial algal mats.

Snottite—a mucous-rich mat of sulfur-oxidizing bacteria (chemosynthetic) found in a sulfuric-acid-rich evironment in Cueva de Villa Luz in Tabasco, Mexico. These organisms could have been some of the earliest life forms on our planet. This cave is full of sulfide springs and microbial draperies reaching nearly 0.5 m in length. These snottites have acid drops at their tips, with a pH ~ 0, and are associated with gypsum and elemental sulfur deposits. This cave system is currently being investigated by a large group of researchers at various institutions. Kenneth Ingham ©

Archaea are bacteria-like organisms that are often tolerant of hot, cold, acidic, or oxygen-free environments. These organisms live in extreme locales like Yellowstone hot springs. All life on Earth may have evolved from Archaea that were able to survive the final, large asteroid impacts of Earth’s early history, perhaps in hydrothermal vents in the ocean floor. Above image and header image are from within Yellowstone National Park. Katryn Wiese ©

Methanogens belong to an ancient group related to bacteria, called the Archaea. They thrive without oxygen. 1,000-m-deep volcanic rocks along the Columbia River and in Idaho Falls host two of these ecosystems. NASA ©

About three km below the ground in a South African gold mine, scientist Duane Moser stands next to the fracture zone (white area) where he and Li-Hung Lin found bacteria that live in an ecosystem driven by radioactive decay with no oxygen, no light, and no organic input. Li-Hung Lin ©

Adult Riftia pachyptila tubeworms at a hydrothermal vent. These worms feed on sulfur-oxididing bacteria—similar to organisms that likely first existed on this planet. Andrea D. Nussbaumer, Charles R. Fisher and Monika Bright ©

A degassing event at Brimstone Pit at NW Rota-1 volcano releases an extraordinary number of bubbles—probably carbon dioxide. The yellow parts of the plume in the back-ground contain tiny droplets of molten sulfur. NOAA ©