Origin of Life Chapter 6.1
Main points Life at the Extremes! The extreme conditions that limit growth, or prove lethal to most organisms can be favorable for others! Extremes that can kill majority of organisms on Earth: Temperature High/low ph High salinity High radiation levels! High T, low ph, high salinity probably persisted throughout Earth history not rare on Earth now either.! Very few natural environments on Earth where life is absent life is a rule rather than an exception!
Evidence towards the origin of life! Start to see evidence in the early Archaean
! The geological record becomes incomplete as we go back in time No known rocks survive from Earth s first 0.5 Gyrs geological records can t tell us how life originated
! However, there are three lines of evidence that life started early on Earth Stromatolites Microfossils Isotopic evidence! If life starts early on, that increases the chance of similar organisms to develop at other worlds, even under less than perfect conditions.
Stromatolites! Layers of sediment that once contained colonies of ancient microbes.! Date back ~3.5 Gyr ago.! Distinctive layered structure.! They contain chemical and isotopic evidence of biological origin. 3.5 Gyr stromatolite, North Pole, Australia
! Size, shape, interior structure ancient stromatolites look identical to mats formed today.
Built up by action of microorganisms:! Layers of sediments intermixed with microbes. Microbes at the top generate energy via photosynthesis Microbes beneath use the organic compounds (waste) of the photosynthetic microbes.! Cyanobacteria: uses carbon compounds for respiration, has a filamentous morphology, makes a slimy sheath! Sulfate reducers: cause carbonate to precipitate glues structure together
! They grow over time as sediments are depositing over, forcing them to migrate upward in order to perform photosynthesis - need the light Microbial mats at Shark Bay, Australia
If the microbes in the ancient stromatolites are like in the modern ones, this means that at least some produced energy by photosynthesis Photosynthesis is a sophisticated metabolic process it took a long time for this process to evolve Therefore primitive life should have already existed some long before 3.5 billion years ago!
Microfossils! Second line of evidence for the early life on Earth, highly controversial.! Microfossil evidence suggests that life existed before about 3.0 billion years ago and may well have existed before 3.5 billion years ago.! Start with the phylogenetic tree, representing the evolutionary relationships amongst a set of organisms
! Time scales: Length of branches is not related to temporal scale, just to evolutionary distance! Some ages included estimated from fossil record 544 Myr 2700 Myr 1200-1000 Myr 2700 Myr 2800 Myr 2700 Myr
! Molecular fossils that are stable under geologic conditions mostly originate from lipids that were once functioning within a living organism (biolipid). Under some conditions, these biolipids are reduced to a biomarker.! The biomarker principle The transformation a biolipid to a biomarker. After burial and millions of years of pressure, the biolipid loses some of its individuality, usually some molecular groups belonging to its chain. The hydrocarbon skeleton left allows geologists and geochemists to recover it and attempt to identify its source.
Example: bacteria containing hopanoid molecule Hopanoids very useful biomarkers since they are not found in Archaea or Eukaryotes After the transformation (biogenesis) the OH group on the chain is lost, and the remaining skeleton is called hopane.
Why microfossils are controversial! Difficulties: Finding old fossils difficult the oldest rocks have been altered by geological processes destroying microfossils. When searching for evidence of past life, sedimentary environments are considered the most suitable because they are often formed in association with water, a fundamental requirement for life. There are only three known locations that host exposures of ancient sediments: Greenland (3.8 to 3.7 Gyr old), in Australia (3.5 to 3.3 Gyr), and South Africa (3.5 to 3.3 Gyr).
! Contamination of ancient rocks by recent endolithic microorganisms further complicates the task of identifying the original signatures of life! Endoliths are organisms that live inside rocks or in pores between mineral grains.! The claim of the discovery of microfossils is controversial because it is not clear whether they are of biological or abiotic origin.
Pioneering work by Bill Schopf, UCLA, who found microfossils in Warrawoona, Australia, implying 3.45 Gyr age. Shapes interpreted as filamentous cyanobacteria, and size consistent (larger than most bacteria) Organic C has isotopic values typically found in microbial biomass Consistent with 1) cyanobacteria and 2) biological source
Martin Brasier, 2002 Nature paper, found the microfossils to be weird The rock had been presumed to be a sedimentary rock from a shallow sea and the fossils of a photosynthetic microbe. Brasier s rocks analysis indicates is from a deep-sea volcanic vent (250-350 ), therefore the fossil cannot be of a photosynthetic microbe. C isotopic fractions could be from abiotic reactions
Abiotic microfossil?
Isotopic evidence Studies of carbon isotopes can be used to detect the presence of past biological activity in rocks.! The earliest evidence for life comes from chemical remnants of biological processes in rocks.! Carbon comes in three varieties: 12 C, 13 C and 14 C. 12 C, is the normal variety, has 6 protons and 6 neutrons 13 C and 14 C have 1 and 2 additional neutrons, respectively! Organisms ingesting carbon prefers 12 C over 13 C 14 C is radioactive and will thus not remain for long times
! Many fundamental biochemical processes involving the extraction of carbon from an abiotic source 12 C most abundant, most reactive, preferred by enzymes in cells E.g,. an enzyme that captures C from CO 2, preferentially removes 12 C from CO 2 because of the weaker carbon-oxygen bond strength in the lighter isotope.! Hence, remnants of organic carbon should be enriched in 12 C/ 13 C relative to the contemporaneous atmospheric CO 2 Carbonate rocks enriched in 13 C
! Each organism produces organic material with a certain range of 13 C! If we find ancient organic carbon we can estimate what organisms produced it
! Carbon isotope evidence from rocks found in Greenland (controversial) suggests that life may have existed 3.85 Gyr ago.! Rocks of similar age (3.8Gyr) show similar C isotope ratios to the Greenland rocks. More than one single rock! Life can also alter isotopic ratios of other elements (Fe, N, S). These isotopes are present in characteristic ratios for life within the ancient rocks, confirming existence of life as early as 3.85 Gyr ago.
! Organic material enriched in 12 C goes back to the earliest rocks.! Suggests an active biosphere with photosynthesis dating back to 3.5 Gyr.