From Molecules to Cells
***note from Erika*** this article was not finished and
needs more work (see First Cells)
How did the first life forms on our planet evolve from a
mere chemical broth? It is a matter of conjecture at what point a complex
assemblage of molecules deserves to be called a life form. We do not know how
and when those first primitive life forms emerged, but scientists have come up
with a likely scenario. It is generally agreed that the appearance of life
depended on the changing conditions on the early Earth. The Earth formed about
4.6 billion years ago, with a primitive atmosphere containing gases rich in
carbon and nitrogen and deficient in hydrogen and oxygen. During its first 100
million years, the Earth was bombarded by many rocky fragments, some of which
may have been as large as the Moon. Such impacts would have vaporized the
oceans and a large amount of the Earth's crust. Material rich in hydrogen,
oxygen, and complex molecules arrived somewhat later, in the form of comets and
objects from the outer solar system. The early Earth was a strange and almost
unrecognizable place, yet life first formed in this environment.
The next step after the first replicating molecules —
aggregating complex organic molecules into cell-like structures — must have
taken place in the 500 million years after the Earth formed. Although some of
the processes can occur in dry environments, liquid water was probably critical
to biochemical evolution because it provided a medium in which materials could
move and stick together. One botanist has commented that "all cells, of
all living organisms, are strictly aquatic creatures." Any land-based
organism is merely a protective shell filled up with millions of aquatic cells.
Biological processes cannot develop unless they are set
apart from the environment and protected from dilution. Some sort of
compartment or membrane is required to form a cell.
The next step towards recognizable life is even more
uncertain. If organic molecules or coacervates are present in a pool of water,
they will be left in the pool as the water evaporates. In this way, evaporation
of the water in tidewater pools provided high, localized concentrations of amino
acids, proteins, and other molecules, allowing cell-like structures to form.
The cell-like structures in the primeval pools of "organic broth"
could have begun reacting with fluids in the pools and with each other,
accumulating more molecules and growing more complex. This concept was first
suggested by Charles Darwin, who speculated on "some warm little
pond" where life might have begun. Eventually these early cells could have
evolved into biochemical systems capable of reproducing and increasing in complexity.
A cell is a sophisticated chemical factory. It is not surprising that we cannot
duplicate this evolution in the laboratory, since it took half a billion years
on Earth.
The earliest biological systems capable of independent life
were bacteria. Bacterial cells are prokaryotes, cells without nuclei that
contain a single long strand of DNA with several thousand genes. Indirect
evidence of bacteria has been found in the Earth’s oldest rocks. The evidence
consists of carbon isotopes of possible biological origin found in a
3.8-billion-year-old rock from western
The fact that the oldest fossils are younger than the oldest
rocks may not be significant. Life may have originated considerably earlier.
However, life probably could not have evolved much before 4.1 billion years ago
because of the intense early meteoric bombardment and the possible ocean of
liquid lava covering much of the Earth's crust. As paleontologist Stephen Jay
Gould has observed, life arose "as soon as it could; perhaps it was as
inevitable as quartz or feldspar." One set of calculations has been used
to argue that life may have originated several times on the early Earth, and
that all life today would have descended from just one of the origination
events. It is also noteworthy that the early Australian stromatolites prospered
in strange and hostile environments. Evidence suggests that these organisms
lived near shallow hydro-thermal vents dominated by island volcanism. In an
atmosphere with almost no oxygen, they metabolized the gas hydrogen sulfide
(H2S), which is toxic to most modern life forms.
Using fossil and chemical evidence, and a little
speculation, we can tell the story of life on Earth. For around 2 billion
years, prokaryotes ruled the oceans of the Earth. Life remained in the oceans,
where liquid water provided a supporting and protective environment. Organisms
were mostly soft-bodied and rarely produced fossils, so their development is
hard to trace. The land was barren. Some areas must have looked like today's
deserts or like Mars. Some areas were moist and washed by rains, but instead of
luxurious forests, there were only bare acres, eroded gullies, and grand
canyons. Brown vistas stretched to the sea.
Gradually, life went through a remarkable transition. Early
prokaryotes survived and evolved by using the organic compounds that were
present in warm ponds and by using hydrogen sulfide as an energy source.
However, as this source of food was extinguished by changes in the Earth's
atmosphere, some prokaryotes invented photosynthesis. Photosynthesis allows the
conversion of sunlight into stored chemical energy for future use. This
fundamental process allowed the proliferation of advanced forms of life and
allowed life to endure for a cosmic time scale. Life's destiny became coupled
to the fusion energy source deep in the Sun's interior.
One result of the invention of photosynthesis was the release
of oxygen into the Earth's atmosphere. Essentially all of the free oxygen on
which modern life forms depend (including us!) was dumped into the atmosphere
by microscopic organisms billions of years ago. At first this gas was nothing
more than a waste product — oxygen was actually poisonous to the first
organisms! Over time, organisms evolved that could use oxygen in their
metabolism, and the oxygen content began to rise towards the modern value of
21%. The atmosphere therefore evolved from more reducing conditions (dominated
by hydrogen compounds) towards oxidizing conditions (dominated by oxygen
compounds). As evidence for this change, we find that oxidized sediments are
rare before 2 billion years ago and common afterwards. Oxygen production
modified the whole environment. Solar UV radiation broke down some O2 molecules
and the free oxygen atoms joined with other O2 molecules to make ozone (O3)
molecules. The result was the formation of an ozone layer high in the Earth's
atmosphere, which absorbs solar UV radiation and thus protects organisms on the
surface.
About 1.4 billion years ago, life stepped upward in
complexity. Cells called eukaryotes developed; a membrane at the center of
these cells holds the DNA. Eukaryotes contain hundreds of times more genetic
material than prokaryotes, with a corresponding increase in the complexity of
cell function. New types of organisms appear in the fossil record: they are
capable of oxygen metabolism, and, although less resistant to UV damage, they
are able to flourish because of the new ozone layer. Earth then witnesses the
expansion of life from the sea onto the land — a step as momentous as the
contemplated colonization of other planets by us! As organisms continued to
reproduce, some of them invented sex. Sexual reproduction allows an offspring
to receive half of its genes from each parent. All eukaryotic cells can
reproduce asexually, but sexual reproduction causes the gene combinations to
change from generation to generation. Such new combinations in turn facilitate
the experimentation and adaptation that allows organisms to survive in a
hostile and changing environment.
(C) Chris Impey and William Hartmann
Recommendations:
THE ORIGIN OF LIFE ON EARTH
EVOLUTION AND INTELLIGENCE
THE NATURE OF LIFE
LIFE AS DIGITAL INFORMATION
NATURAL SELECTION
ENVIRONMENTAL CHANGE ON EARTH
THE CREATION OF HEAVY ELEMENTS
STATES OF MATTER