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Feature Article
Tracking Down
Early Evolution
of Life
Tracking fossil records gives an interesting
insight into how life evolved
DEEPAK JOSHI
M
OST people are aware that fossils
(preserved life in historic rocks) are
remains of the animals and plants
that once lived on the Earth. But, few know
that fossils are crucial keys that reveal our
relation to that primitive cell, which most
likely formed near about 3.5 billion years
ago. Life as it exists today, evolved from
that first mother cell.
The Earth was formed about 4.5 billion
years ago and life began on the Earth over
3.5 billion years ago. Since the beginning,
life has achieved three major steps in its
evolution. The first was appearance of
prokar yote organisms (single cell, cells
devoid of nucleus, and DNA not arranged),
then came eukaryotes (highly organized
single cell, with nucleus within which reside
the chromosome with arranged DNA) and
finally multicellular organisms (combination
of eukaryote cells). This evolution mainly
occurred in the Precambrian time of the
Earth’s history.
The entire geological time scale is
mainly divided in two parts, Precambrian
and Phanerozoic. The Precambrian is
known as the age of “microscopic life”
and Phanerozoic for “megascopic life”.
The Cambrian time has great relevance in
the Earth’s histor y because this was the
time when life exploded on the Earth.
Cambrian onwards rock sequences
preserved abundant fossils of marine and
land life. Since the formation of the Earth
up to the base of the Cambrian at 542
million years ago (Ma), the period is known
as Precambrian.
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Skeletalisation event was the key to the evolution
of life from the early Cambrian arthropods to the
giant dinosaurs and then on to humans.
Let us track the fossil records with
respect to the geological time, providing
us a picture how life traveled its
evolutionar y journey from single cell to
modern complex multicellular life.
Oldest Fossil Record
Earth and life evolved during the Archean
period. This was the time when crust was
formed on the Earth’s surface ~ 4.0 billion
years ago. Since Archean time, rocks have
preserved history of evolution of life. At
present, the world’s oldest fossils are known
from the 3.3 to 3.6 billion years old
Warrawoona Group rocks of Western
Australia. Stromatolite and tiny filamentous
fossils have been reported from these
rocks.
Stromatolites are bio-sedimentar y
structures formed due to trapping and
binding of the sediments and metabolic
activity of microorganisms mainly by bluegreen algae. With the presence of
stromatolite and filamentous type forms in
the middle Archean (3.3-3.6 billion years)
we can assume the existence of life at this
level.
Proterozoic stromatolites
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Appearance of Prokaryotes
The most significant feature of Archean
time is arrival of life. Scientists are agreed
that both the essentials for the origin of life
were present during this time: first, elements
suitable for life and secondly energy source
that could synthesize organic molecules.
It is perceived that organisms are mainly
composed of carbon, hydrogen, nitrogen
and oxygen. These components were
present in the early atmosphere of the Earth
in the form of CO2, H2O, CH4 and NH3. The
term ‘monomers’ is used for a combination
of simple organic molecules containing
mainly C, H, N, and O. Experimental
evidence for the formation of monomers
has been shown by Miller in 1953 who
synthesized several amino acids through
the mixture of some gases that existed in
the early atmosphere of the Earth.
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Negaunee Iron Formation in Michigan—This striking
example of banded iron formation is more than 2 billion
years old
So, the earliest organic molecules
might have been synthesized from
atmospheric gases of the early Archean
and subsequently the first cell (without
nucleus) might have been formed. But this
explanation of origin of life has been
questioned by some workers, who propose
that life originated via the hydrothermal
vent systems on the seafloor. According
to this hypothesis, ocean water that
contained large quantities of dissolved
minerals percolated inside the rocks
through cracks and fissures, and was
heated by hot magma, and then again
came to the surface through hydrothermal
vents in a complex form including organic
molecules.
As per general understanding,
elements and energy are required for the
synthesis of molecules; the hydrothermal
vent system fulfills both requirements. Even
amino acids have been discovered in
modern hydrothermal vent systems on the
sea floor. Through this complex process
protocell would have been formed in the
Archean.
The earliest living organisms on the
Earth were undoubtedly prokaryotes and
primitive prokaryotes were bacteria and
cyanobacteria (blue-green algae). The
filamentous forms have been reported
from the Warrawoona Group rocks of
Australia, which is considered to be 3.5
billion years old. This fossil record is
considered as the oldest evidence of
prokar yote type of life. However, their
presence in the Archean was scarce but
they thrived during the Paleoproterozoic
(2500-1600
Ma).
Well-preserved
prokar yotes have been documented
from the 2 billion years old Gunflint chert
of Precambrian Iron Formation of
Canada.
Concepts of evolutionary trend within
the prokaryotes have also been proposed
by some workers mainly by the J. W.
Schopf of California University. In evolution
of bacteria, anaerobic bacteria came
first than aerobic bacteria and this
evolutionar y change happened in
between 3.5 to 2.5 billion years ago
(mainly in Archean time). Since it has been
revealed by some studies that primitive
atmosphere had less O2 or may be totally
absent, therefore, anaerobic bacteria
may have appeared before the aerobic
bacteria.
Appearance of Eukaryotes
All living organisms, other than prokaryotes,
are eukaryotic. Eukaryotic cells are higher
in organization than prokar yotic cells.
Present paleobiological studies indicate
that they evolved during the Proterozoic
time. Cyanobacteria (aerobic) increased
oxygen level in the primitive atmosphere
through the process of photosynthesis and
it is thought that this presence of free
oxygen in the atmosphere triggered the
evolution of eukaryotes.
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Since prokaryotes appeared much
earlier than eukaryotes, it is thought that
eukar yotes could have evolved from
prokaryotes. A current prevailing concept
among the evolutionary biologists holds
that “ several prokar yotes make a
eukaryote”. This denotes symbiotic relation
between the prokaryote cells. Symbiosis is
a phenomenon in which two or more
dissimilar organisms coexist with mutual
benefits. For example, lichen, once
considered to be plants, are actually
symbiotic fungi and algae.
Lynn Margulis of Boston University
proposed that mitochondria and
chloroplast are important constituents of
eukaryote cell. In support of this symbiotic
relationship it has been observed that both
mitochondria and chloroplast contain a
small fragment of DNA whose organization
is similar to prokaryotes.
In fossil records prokar yotes
appeared in the middle Archean and
diversified 2 billion years ago. On the other
hand, the oldest known occurrence of the
eukaryotic organism is known from the ~2
billion years old Negaunee Iron Formation
of Michigan, Canada and reported fossil
known as Gr ypania. Some speculative
eukaryotic organisms are also known from
the rocks of McArthur Group (1600-1700
Ma old).
Multicellular Organisms
In the evolutionar y fossil records of
eukaryotes, it is very difficult to distinguish
between the appearance time of single
and multicellular eukar yotes. However,
some metaphytes (multicellular algae)
have been documented from the 1.7
billion year old Tuanshazi rock of the Jixian
area of North China. Accordingly, we can
assume that the first unicellular eukaryotic
organisms evolved ~2 billion years ago
followed by multicellular eukar yotes
~1700 million years ago. Multicellular
organisms are composed of many cells
of eukar yotes. Accordingly, in the
evolutionary order of life we can propose
that unicellular eukar yotes gave rise to
multicellular organisms.
Multicellularity evolved among
unicellular organisms because: i)
multicellularity increased the size, due to
which less number of cells are exposed to
the external environment, ii) process of
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Ancient fossils have been found in the Warrawoona
group of rocks
material transfer from the external
environment into the cell became more
easy, iii) cells could imitate, to increase
competency of respiration, and iv) most
importantly, some specialized cells
increased the efficiency for particular tasks
as they converted into organs.
Appearance of Metazoans
The appearance time of metazoans is a
most intriguing question for biologists and
paleontologists. Several types of studies
have been made to sort out this problem,
especially on the basis of molecular
studies. Amino acid changes occurred
during the divergence between species
and this process happened at a constant
rate. This study suggests that molecular
evolution is a constant process and
provides a molecular clock.
Through this study we can deduce the
amount of molecular change between
two species and then can measure how
long ago they belonged to a common
ancestor. Fossil records and radiometric
dates of fossil-containing rock further
constrain the molecular clock. It has been
suggested through this study that
multicellular animals evolved ~ 700 million
years to more than 1.5 billion years ago,
but evidence of their appearance in fossil
Complex organic molecules from 350-million-year-old
fossil sea creatures
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records is documented from rocks younger
than 600 million years.
of India also correspond with this great
historical event.
Explosion of Life
Appearance of Hard
Skeleton
Animal life achieved its multicellularity ~
600 million years ago, but at that time life
was ver y simple and only soft bodied
worm-like creatures were present. The
Ediacaran period (635-542 Ma) in which
this soft bodied life existed, closed ~ 542
million years ago. The Precambrian era
also ended with the Ediacaran time, and
the Phanerozoic era started from here.
The major criterion that is used to
distinguish both the time frames is abrupt
presence of life from Cambrian onwards.
The sudden appearance of major animal
phyla in fossil records is known as
“Cambrian explosion”. This life explosion
brought calcareous algae, sponges,
molluscs and echinoderms etc. on the
Earth, most of them extant up to the
present. So, the major animal phyla on
Earth appeared ~ 542 million years ago.
Now, what happened on the Earth in
the Cambrian time due to which life
exploded and diversified? There are lots
of explanations and hypotheses.
During transition time between
Ediacaran and early Cambrian (635-542
Ma), the existing atmosphere was also in
an evolutionar y stage. The significant
change was build-up of free oxygen in
the atmosphere as a result of
photosynthesis process, which finally
attained a level sufficient to support fast
evolutionary process, resulting in the bloom
in animal life.
Marine transgression (rise of sea level),
which happened immediately after the
latest Neoproterozoic (635 Ma) glaciation
(ice sheet) event due to warming of
climate, opened new shallow water
ecological niches (life favorable
environment) near the shelf zones of
continents. Accordingly the availability of
food increased.
Due to this marine transgression,
phosphorous accumulated in the deep
sea was brought up near the shelves thus
also increasing the nutrient level.
Phosphorus is essential for life and this event
triggered the sudden diversification of
animal life. Accordingly the deposits of the
phosphorite along the PrecambrianCambrian boundar y are recorded the
world over. Phosphorite deposits in the
Lesser Himalaya region (mainly in and
around Mussoorie township of Uttarakhand)
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At what time did life get its skeletal part,
and what were the reasons behind it?
Interestingly, the diversification of life and
its skeletalisation were both a simultaneous
event. Skeletalisation event has also been
dated ~542 million years ago. The only
minor difference in both events is that life
diversified highly during 530-520 million
years ago, but obtained skeletalisation a
little earlier, ~ 548-542 million years ago.
We can assume that the skeleton gave
protection to life and due to this life
diversified later.
Some studies show that eukar yotic
cells are able to produce protein
substances, capable of mineralization.
Another view regarding skeletalisation is
that, the calcareous or phosphatic
material originated as an excretor y
product, which was accumulated over the
skin and hardened it. Some organisms have
mechanical tendency to make their own
envelope through available surrounding
material, called agglutination; this also
served as the primary skeleton.
Shells (hard armour) of Cambrian
animals were mainly composed of
calcium phosphate and calcium
carbonate, and so large accumulation of
both substances along the PrecambrianCambrian boundary was a possible factor
leading to hardening of animal shells. It is
also suggested that the phosphate
excreting process was developed in
animals at the time of high phosphate
availability in the ocean. Some studies
indicate that oxygen level that was
increasing during Precambrian-Cambrian
transition enhanced the producing power
of skeletal mineral (organic) and protein.
Once the hard part developed,
organisms used it as protection, support,
friction against surrounding hard things,
and increasing muscle power. Finally,
skeletalisation event was the key to the
evolution of life from the early Cambrian
arthropods to the giant dinosaurs and then
on to humans.
Dr. Deepak Joshi is Consultant Exploration
Geologist-Schlumberger, 122, Indira Colony,
Chukhuwala, Dehra Dun-248001, Uttarakhand;
Email: joshides@rediff; Mob.: 9987886184