Eps Sci 3 Lecture 12. Fossils of Primitive Life Forms on Earth

Fossil

preserved remains or traces of organisms from the distant past

3 main types of fossils

body fossils, trace fossils (ichnofossils), and chemofossils

Trace fossil

evidence of an organism’s activity, such as footprints or fossilized feces

chemofossil

a biochemical signal or biomarker indicating possible past life

why is fossilization exceptionally rare?

most organic remains decompose before they can be preserved

conditions that favor fossil preservation;

rapid burial, freezing, desiccation, and some anoxic environments

which organisms are most likely to be fossilized?

those with hard parts, widespread populations, and long geological existence

which are more common: macrofossils or microfossils?

microfossils, although macrofossils are more commonly displayed

why are microbial fossils difficult to identify

microbes are tiny, lack hard parts, and resemble abiotic mineral structures

6 biomarker categories

cells, sediment textures, organic matter, minerals, isotopes, atmospheric gases

biogenic minerals

minerals whose formation or deposition was influenced by biological processes

what does light microscopy reveal

morphology and spatial organization; resolution and chemistry are limited

what does electron microscopy reveal

high-resolution morphology and ultrastructure; not biological origin alone

What does Raman spectroscopy identify>

organic compounds, pigments, and minerals

GC-IRMS

stable isotope ratios, such as carbon isotope ratios

why can’t one analytical technique prove microbial life

abiotic processes can mimic biological structures and chemical signatures

how did Xiao et al. (1998) interpret 570-Myr fossils?

as fertilized animal eggs, potentially indicating early animal groups

how did Bailey et al. (2006) interpret those fossils?

as possible ancient sulfur bacteria resembling Thiomargarita

microbial mat

layered community of microorganisms growing together on a surface

what evidence supported 3.2-Ga microbial mats in South Africa

modern mat-like textures and δ¹³C values of −20.1‰ to −21.5‰.

Why can negative δ¹³C values suggest life?

Biological processes often favor carbon-12, producing isotopically light carbon.s

stromatolite

layered sedimentary structure that can form through microbial activity

possible 3.4-Ga stromatolites studied

Australia, in the study by Allwood et al. (2006)

2 very early life discoveries

possible 4.1-Ga biogenic carbon and 3.77-4.28-Ga hydrothermal vent evidence

important Archean rocks preserved

Pilbara craton in western australia and kaapvall craton in south africa

ALH84001

a martian meteorite investigated for possible evidence of ancient life

3 features of ALH84001 that suggested possible life

carbonate globules, PAhs, and magnetosome-like magnetite structures

why don’t carbonate globules prove martian life

they can form through nonbiological processes

why don’t PAHs prove Martian life

they also occur in asteroids, comets, meteorites, and lifeless space

magnetosomes

magnetic mineral structures produced by certain bacteria

what abiotic process could explain ALH84001 magnetite?

shock heating of carbonate material, allowing magnetite grains to form

main conclusion about extraterrestrial fossils

suggestive features are not proof; abiotic explanations must be evalauted