Definition of life’s origins and early evolution

The origin of life and its early evolution from simple single-celled organisms to

more complex forms has intrigued scientists, philosophers,

theologians, and people from all parts of the world for much

of recorded history. The question of the origin of life relates

to where we came from as humans, why we are here, and

what the future holds for our species. It is one of the most

interdisciplinary of sciences, encompassing cosmology, chemistry,

astrophysics, biology, geology, and math.

Several ideas about the location of the origin of life have

received the most support from the scientific community.

Some scientists believe that life originated by chemical reactions

in a warm little pond, whereas others suggest that it

may have started at surface hot springs. Another model holds

that the energy for life was first derived from deep within the

Earth, at a hydrothermal vent on the seafloor. Still others

hold that life may have fallen to Earth from outer reaches of

the solar system, though this does not answer the question of

where and how it began.

Life on the early Earth would have to have been compatible

with conditions very different from what they are on

the present-day Earth. The Earth’s early atmosphere had no

or very little oxygen, so the partial pressure of oxygen was

lower and the partial pressure of CO2 was higher on the

early Archean. The Sun’s luminosity was about 25 percent

less than that of today, but since the early atmosphere was

rich in CO2, CH4 (methane), NH3 (ammonia), and N20

(Nitrous oxide), an early greenhouse effect warmed the surface

of the planet. CO2 was present at about 100 times its

present abundance, so the surface was probably even hotter

than today, despite the Sun’s decreased luminosity. Evidence

suggests that the surface temperature was about 140°F

(60°C), so thermophyllic bacteria (heat adaptive) were

favored to evolve over organisms that could not tolerate

such high temperatures. The lack of free oxygen and radiation-

shielding ozone (O3) in the early atmosphere led to a 30

percent higher ultraviolet flux from the Sun, which would

have been deadly to most early life. The impact rate from

meteorites was higher, and heat flow from the interior of

Earth was about three times higher than at present. Early life

would have to have been compatible with these conditions,

so it would have to have been thermophyllic and chemosynthetic.

The best place for life under these extreme conditions

would be deep in the ocean. The surface would have been

downright unpleasant.

Since the Earth is cool today, some process must have

removed CO2 from the atmosphere, otherwise it would have

had a runaway greenhouse effect, similar to Venus. Processes

that remove CO2 from atmosphere include deposition of

limestone (CaCO3), and burial of organic matter (CH2O).

These processes are aided by chemical weathering of silicates

(e.g., CaSiO3) by CO2 rich rainwater, that produces dissolved

Ca2+, SiO2, and bicarbonate (HCO3

–), which is then deposited

as limestone and silica. Life evolved in the early Precambrian

and began to deposit organic carbon, removing CO2

from the atmosphere. Limestones formed as a result of organic

processes acted as big CO2 sinks and served to lower global

temperatures.

The present-day levels of CO2 in the atmosphere are balanced

by processes that remove CO2 from the atmosphere

and processes that return CO2 to the atmosphere. Today

78,000 billion tons of carbon are stored in sedimentary

rocks. It would take a few hundred million years to accumulate

this from the atmosphere. The return part of the carbon

cycle is dominated by a few processes. The decomposition of

organic matter releases CO2. Limestone deposited on continental

margins is eventually subducted, or metamorphosed

into calc-silicate (CaSiO3) rocks, both processes that release

CO2. This system of CO2 cycling regulates atmospheric CO2,

and thus global temperature, on long time scales. Changes in

the rates of carbon cycling are intimately associated with

changes in rates of plate tectonics, showing that tectonics,

atmospheric composition and temperature, and the development

of life are closely linked in many different ways.

It is often difficult to recognize signs of life in very old

deformed rocks. Signs of life in rocks are often detected by

searching for geochemical isotope fractionation. Metabolism

produces distinctive isotopic signatures in C—there is about a

5 percent difference between 13C/12C of organic v. inorganic

carbon. So, the presence of isotopically light carbon in old

rocks suggests the influence of life. However, life may have

been diverse, photosynthesizing, methanogenic, and methylothropic

bacteria by 3.5 billion or even 3.85 billion years

ago. Early life, in a pre–oxygen-rich atmosphere, had to be

adapted to the reducing environment.

Early life consisted of 3.8 Ga primitive bacteria (prokaryotes

means before nuclei), that may have been photosynthetic.

These bacteria made food from CO2, water, and energy from

the Sun, but did not release O2. Many of the bacteria oxidized

sulfur from S2

– to sulfate SO4

2–. Oxygen is toxic to these bacteria,

so they must have lived in environments with no oxygen,

reducing the sulfate ion for their energy. By 3.5 Ga

cyanobacteria, or blue-green algae, used CO2 and emitted

oxygen to the atmosphere. These organisms began to form the

protective ozone (O3) layer, blocking UV from the Sun and

making the surface habitable for other organisms.

Recently, 2.5-billion-year-old Archean ophiolites with

black smoker types of hydrothermal vents and evidence for

primitive life-forms have been discovered in northern China.

The physical conditions at these and even older mid-ocean

ridges permit the inorganic synthesis of amino acids and

other prebiotic organic molecules, and this environment

would have been sheltered from early high ultraviolet radiation

and many effects of the impacts. In this environment, the

locus of precipitation and synthesis for life might have been

in small iron-sulfide globules emitted by hydrothermal vents

on the seafloor. Right now, this day, we have very primitive

bacteria on the East Pacific Rise on black smoker chimneys,

living at 230°F (110°C), the highest temperature in which life

exists on Earth. Life at the black smokers draws energy from

the internal energy of the Earth (not the Sun), via oxidation

in a reducing environment.

Life apparently remained relatively simple for more than

a billion years. However, sometime roughly around 2.5 billion

years ago, life changed from primitive prokaryotes to eukaryotes,

with cell nuclei. Molecular biology yields some clues

about life at 2.5 Ga. Molecular phylogenies compare genetic

sequences and show that all living species cluster into three

groups, Archea, Bacteria, and Eukarya (plants and animals).

They all have a common ancestor that is thermophyllic, or

heat-loving. So, the deepest branches of the “Universal Tree of

Life” are dominated by heat-loving species. This amazing fact

suggests that hydrothermal systems may have been the location

for the development of early life. Furthermore, the oldest

thermophiles are all chemosynthetic organisms that use H and

S in their metabolism. H and S are readily available at the

black smoker chimneys, adding further support to the idea

that submarine hydrothermal vents may have been the site of

the development of Earth’s earliest life.

Late Archean (2.5 Ga) BIF (banded iron formation) associated

with the Dongwanzi, Zunhua, and Wutai Shan ophiolites

in North China have black smoker chimneys associated

with them, and some of these bear signs of early life. This is a

time when the Earth surface environment began a dramatic

shift from reducing environments to highly oxidizing conditions.

This may be when photosynthesis (i.e., the metabolic

strategy common today) developed in sulfur bacteria. Oxygenic

photosynthesis first developed in cyanobacteria and

later transferred to plants (Eukaryotes) through an endosymbiotic

association.

Life continued to have a major role in controlling atmospheric

composition and temperature for the next couple of

billion years. The first well-documented ice age is at the

Archean/Proterozoic boundary, although some evidence

points to other ice ages in the Archean. The Archean/Proterozoic

ice age may have been related to decreasing tectonic

activity and to less CO2 in the atmosphere. Decreasing plate

tectonic activity results in less CO2 released by metamorphism

and volcanism. These trends resulted in global levels of atmospheric

CO2 falling, and this in turn caused a less effective

greenhouse, enhancing cooling, and leading to the ice age.

Prolonged cool periods in Earth history are called icehouses,

and most result from decreased tectonic activity and

the formation of supercontinents. Intervening warm periods

are called hothouses, or greenhouses. In hothouse periods,

higher temperatures cause more water vapor to be evaporated

and stored in the atmosphere, so more rain falls during

hothouses than in normal times, increasing the rates of chemical

weathering, especially of calcium silicates. These free Ca

and Si ions in the ocean combine with atmospheric CO2 and

O2, to form limestone and silica that gets deposited in the

oceans. This in turn causes increased removal of CO2 from

the atmosphere, which cools the planet in a self-regulating

mechanism. The cooling reduces the rate of chemical weathering.

Eventually, previously deposited calc-silicates are

buried, metamorphosed, and release CO2, which counters the

cooling from a runaway ice age effect, warming the planet in

another self-regulating step.

The earliest bacteria appear to have been sulfate-reducing

thermophilic organisms that dissolved sulfate by reduction to

produce sulfide. In this process the bacteria oxidize organic

matter, to produce CO2 and H2O, and deposit FeS2 (pyrite).

Banded iron formations (BIFs) are rocks rich in iron and

silica and are very common in 2.2 to 1.6 Ga old rock

sequences. They are the source of 90 percent of the world’s

iron ore. BIFs were probably deposited during a hothouse

interval and require low oxygen in the atmosphere/hydrosphere

system to form. It is hypothesized that water with

high concentrations of Fe2+ was derived from weathering of

crust.

Eukaryotes with membrane-bound cell nuclei emerged at

about 2 Ga. Aerobic photosynthetic cells evolved and very

effectively generated oxygen. These organisms rapidly built

up atmospheric oxygen, to high levels by 1.6 Ga. The eukaryotes

evolved into plants and animals. For the next billion

years, oxygen increased and CO2 fell in the atmosphere until

the late Proterozoic, when the explosion of invertebrate metazoans

(jellyfish) marked the emergence of complex Phanerozoic

styles of life on Earth. This transition occurred during

the formation and breakup of the supercontinent of Gondwana,

with associated climate changes from a 700 Ma global

icehouse (supercontinent), with worldwide glaciations, to

equatorial regions. This was followed by warmer climates

and rapid diversification of life.

See also ARCHEAN; BANDED IRON FORMATION; CARBON

CYCLE; HADEAN.

limestone See CHEMICAL SEDIMENT; SEDIMENTARY ROCKS.

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