How Long the Life has Existed on Earth ?
– planet earth ~4.550 billion years old, known from decay rate of radioactive elements preserved in earth’s rock strata.
oldest rocks from Greenland,3.8 billion years old.
- 3.36 billion years old from Australia.
- 3.60 billion years old from South Africa.
In 1977, analysis of a 3.40 billion years old rock from South Africa ‘Chertz’-formed by “mineral-laden volcanic water poured over thick mud”;
microscopic section showed minute round, dumbbell shaped structure look like modern day bacteria, this lead searches from other rocks
all yielded microfossils, single celled life with increasing diversity in younger rocks.
Gun Flint (Canada, 2.5 billion) yielded spheres, rods, filaments, stalked blobs & tentacled forms.
The very oldest rocks known do not harbor micro- fossils.
The ratio of carbon isotopes, C12 , C13 , in the rocks is unusual with higher C12 than expected if abiotic (non-living) inorganic chemistry processes were solely responsible.
The Age of Bacteria
From ancient origins to 670 million years ago biodiversity was dominated by bacteria & their kin, a Kingdom of life called Prokaryotes.
Prokaryotic evolution is the foundation for the genetic, metabolic and ecological diversity of today. Their role in earth’ s history shows the power of living things to alter the environment at planetary scale.
The precise environmental chemistry of the Archean environment though disputed:
but general characteristics were an atmosphere of N, H, CO2, water vapour & traces of other gases with the exception of O2.
The lack of O2 in ancient seas created reduced environment, determined forms of chemical ions present.
The Archaic environment provided bases in which Prokaryotes developed a diversity of metabolic systems for 3 purposes-
- releasing energy;
- building Carbon reserves;
- tolerating extreme environments.
Archean microbes able to survive at wide range of hostile habitats like, strongly alkaline soda lakes, thermal vents discharging water hotter than 100°C, desiccating salt pans, etc.
Many of these tolerant bacteria now known as Archeabacteria separated from other bacteria called Eubacteria & these 2 domain combine to form Prokaryotes.
All other life belongs to a 3rd domain Eukaryota.
Eukaryotes are distinguished by the biochemistry of structures inside their cells called ribosome, sites where genetic code is read & used to build protein.
Archean bacterial diversity flourished but in lack of gaseous oxygen,
free oxygen toxic to many bacteria, highly reactive and readily combines with other common elements.
Atmospheric oxygen: massive departure from chemical equillibrium,
free oxygen originates also from life, an example of biodiversity changing global environment and the 1st & most severe biodiversity crisis afflicting earth.
Oxygen originates as by-product from photosynthetic splitting of H2O to obtain H ions.
This atmospheric revolution brought about by another group called Cyanobacteria, photoautotrophic Prokaryotes.
Cyanobacteria (BGA)- oldest algae with definite fossil remains & dates back to 3.0-2.8 bio years; as Stromatoliths.
Atmosphere contained little or no oxygen, methane, ammonia & other reduced compounds prevalent Archaeologists barbertonensis, known BGA of that time.
From then until about 1.8 million years O2 production mopped up, reacting with abundant Ca & iron in sea; some 2.0 billion years old thick iron band deposits were evident.
The replacement of CO2-CH4 rich ‘green house’ atmosphere with O2 may have sparked the 1st known ‘Ice Age’ for many prokaryotes O2 was a deadly poison;
Global ecosystems were destroyed,
remnant communities banished to habitats beyond O2 reach such as waterlogged mud & deep sea.
For others this was an opportunity using O2 in respiration to release energy (aerobic) more efficient than anaerobic prokaryotes able to use O2 thrived,
in the wake of the O2 holocaust, 3rd domain Eukarya appeared, result of extraordinary biodiversity link up
pro- & eukaryotes differ-
- in prokaryotes, cellular material are scattered;
- in eukaryotes, cellular material in discrete membrane.
Bound structures different organelles specialized in different tasks,
Two the most distinctive are-
- mitochondria- site of aerobic respiration
- chloroplast- site for conversion of solar energy to stored chemical energy.
Both chloroplast & mitochondria have their own genetic information separate from that of the rest of the cell.
When cell divides, mitochondria & chloroplast duplicate themselves simultaneously & separately.
Mitochondria & chloroplasts appeared to be remnants of once independent bacteria, subsumed into a 3rd bacterium, the main body cell.
In addition, many motile hair like cilia outside many animal cells uniquely similar to structure of motile spiro-bacteria.
Hints available of this origin of Eukaryotic cells termed Endosymbiosis among modern creatures.
Most extraordinary example –
Mixotricha paradoxa, collection of at least 5 types of pro- & eukaryotes, live in hind guts of Australian termites Mastotermis darwinensis.
prokaryotic biodiversity characterized by their varied metabolic skills,
origin of O2 was the result of prokaryote photosynthesis, even most ancient smallest life was capable of modifying planetary environment.
A global biodiversity catastrophe due to O2 toxicity and climate change occurred but even this most terrible event did not wipe out life.
O2 tolerant prokaryotes took over, new cellular organisms developed from an intricate association of prokaryotes.