History of the Earth and of life · The history of life
The Origin and Evolution of Life
Before you start, make sure you know: The Origin of Life (Secondary Year 2)
In Cuatro Ciénegas, Coahuila, there are pools in the middle of the desert where mats of microorganisms live and form rocks called stromatolites, similar to those left by the first communities of life more than 3,000 million years ago. No experiment has created life in a test tube, and what happened back then was a gradual process, not a magical instant. Science does not claim to prove exactly how life arose, but to narrow down, with chemistry and fossils, how it could have arisen.
The Early Earth and Prebiotic Synthesis
Very old rocks contain unoxidized iron minerals and sulfur isotopes that show an atmosphere without free oxygen, along with traces of organic carbon. How could the first biomolecules form on a hostile planet with no living things to make them?
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Summarise Oparin and Haldane's heterotrophic hypothesis on the origin of life, and explain what the Miller-Urey experiment consisted of, what they obtained, and what it…
Summarise Oparin and Haldane's heterotrophic hypothesis on the origin of life, and explain what the Miller-Urey experiment consisted of, what they obtained, and what it demonstrated.
A deep-sea bacterium, a yeast used to make pulque, and a human cell share the same genetic code of 64 codons and the same 20 amino acids. Why do organisms so different work with the same biochemical "grammar"?
The milestones in the history of life are ordered with approximate dates: first prokaryotic cells and stromatolites (about 3,500 million years ago, Ma), Great Oxidation Event (2,400 Ma), first eukaryotic cells (around 1,500 Ma), Cambrian explosion (541 Ma), and colonization of land by plants (about 470 Ma). The oxygenic photosynthesis of cyanobacteria began before the Great Oxidation Event, because oxygen took a long time to accumulate in the atmosphere.
What the rocks tell us: banded iron and red beds
The Great Oxidation Event cannot be "seen" directly: it is read in the rocks. In the oxygen-free ocean there was a lot of iron dissolved as $\mathrm{Fe^{2+}}$ (soluble). When cyanobacteria released oxygen, they oxidized it to $\mathrm{Fe^{3+}}$, which forms insoluble oxides and settles to the bottom. This is how banded iron formations (BIFs) formed: alternating layers of iron oxides and silica, very abundant between about 2,700 and 2,400 Ma. The iron in the ocean acted like an oxygen sponge.
While oxygen was being used up this way, it did not reach the atmosphere. That is why there is a lag of hundreds of millions of years between the first cyanobacteria and the accumulation of oxygen in the atmosphere. Oxygen-sensitive minerals serve as witnesses: uraninite ($\mathrm{UO_2}$) and detrital pyrite ($\mathrm{FeS_2}$) oxidize and dissolve in an oxygenated atmosphere, so if they appear as grains carried by rivers, the air still had no free oxygen.
When the ocean's iron ran out, oxygen accumulated. Then BIFs almost disappear and continental red beds appear, rich in $\mathrm{Fe^{3+}}$ oxides, because the soil exposed to the air oxidizes (like the iron of a gate, hence the color). At the same time, the diversity of anaerobic microfossils falls.
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What are stromatolites and which organisms form them? Explain why they are considered one of the oldest pieces of evidence of life on Earth.
What are stromatolites and which organisms form them? Explain why they are considered one of the oldest pieces of evidence of life on Earth.
Place these milestones in the history of life in chronological order and justify the chosen order: appearance of oxygenic photosynthesis, first prokaryotic cells,…
Place these milestones in the history of life in chronological order and justify the chosen order: appearance of oxygenic photosynthesis, first prokaryotic cells, Cambrian explosion, first eukaryotic cells, land colonisation by plants.
About $2\,400$ million years ago, oxygen began to accumulate in the atmosphere (the Great Oxidation Event). Explain which organisms produced it and describe two…
About $2\,400$ million years ago, oxygen began to accumulate in the atmosphere (the Great Oxidation Event). Explain which organisms produced it and describe two consequences of this change for life and for the Earth's surface.
The Origin of the Eukaryotic Cell: The Endosymbiotic Theory
In the diagram, 1 is the host cell, 2 are the bacteria approaching to be engulfed (one aerobic and one photosynthetic), and 3 is the resulting eukaryotic cell, with a mitochondrion and a chloroplast.
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The diagram represents the endosymbiotic theory of the origin of eukaryotic cells proposed by Lynn Margulis. Explain what occurs between cells 1 and 2 to give rise to…
The diagram represents the endosymbiotic theory of the origin of eukaryotic cells proposed by Lynn Margulis. Explain what occurs between cells 1 and 2 to give rise to cell 3, and cite three pieces of evidence supporting this theory related to mitochondria and chloroplasts.
In a Precambrian geological borehole, strata between 2700 and 2100 Ma are analysed. In the lower levels (2700-2400 Ma), abundant banded iron formations (BIFs) with…
In a Precambrian geological borehole, strata between 2700 and 2100 Ma are analysed. In the lower levels (2700-2400 Ma), abundant banded iron formations (BIFs) with uraninite and detrital pyrite (minerals unstable in the presence of O2) are recorded, alongside cyanobacterial biosignatures. Around 2200 Ma, BIFs almost disappear and the first continental red beds (rich in Fe³⁺ oxides) emerge, while the fossil record shows a drastic drop in the diversity of anaerobic microfossils.
a) Justify why there was a lag of hundreds of millions of years between the appearance of the first oxygenic photosynthetic organisms and the effective oxygenation of the free atmosphere.
b) A student states: "Oxygen was a toxic waste product that nearly wiped out the biosphere, but without it, complex eukaryotic organisms would never have been energetically viable." Critically evaluate this statement by comparing the energy balance (ATP yield) and the biochemical adaptations required to cope with oxidative stress.
In ocean trenches there are hydrothermal vents at more than $300\ ^\circ\mathrm{C}$ with thriving ecosystems without sunlight, and the Murchison meteorite revealed more than 70 amino acids from space. Could life have arisen far from the surface of the early seas?
Alkaline hydrothermal vents: a natural "battery"
In some low-temperature (alkaline) hydrothermal chimneys, the fluid that comes out of the subsurface is alkaline (pH 9 to 11) and rich in dissolved hydrogen ($\mathrm{H_2}$). It meets the water of the early ocean, which was acidic (pH 5.5 to 6) and saturated with $\mathrm{CO_2}$ and dissolved $\mathrm{Fe^{2+}}$. The two solutions were separated by very thin walls of iron sulfides ($\mathrm{FeS}$) full of micropores.
In that scenario, the $\mathrm{H_2}$ in the fluid (which donates electrons) and the $\mathrm{CO_2}$ in the ocean (which accepts them) were on opposite sides of the wall. The iron sulfides and the iron in the walls acted as inorganic catalysts, and the micropores concentrated the reactants and protected them from dilution. The energy of the pH gradient and the reaction between $\mathrm{H_2}$ and $\mathrm{CO_2}$ could favor the formation of simple organic molecules, without sunlight and without prior life.
It is a hypothesis, not a demonstrated fact: it is supported by models and laboratory experiments, and it does not by itself explain how heredity or the membrane arose.
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Compare the "primordial soup" hypothesis with the panspermia hypothesis: how they agree, how they differ, and what problem the second leaves unresolved.
Compare the "primordial soup" hypothesis with the panspermia hypothesis: how they agree, how they differ, and what problem the second leaves unresolved.
A hydrothermal simulator replicating the Hadean ocean floor has been designed in the laboratory: alkaline fluids rich in dissolved H2 (pH between 9.0 and 11.0) emerge…
A hydrothermal simulator replicating the Hadean ocean floor has been designed in the laboratory: alkaline fluids rich in dissolved H2 (pH between 9.0 and 11.0) emerge through mineral microcavities of iron sulphide (FeS) in contact with an acidic primordial ocean (pH between 5.5 and 6.0) saturated with CO2 and dissolved Fe²⁺.
a) Justify how the inorganic microporous structure and the pH difference between fluids abiotically reproduce a mechanism analogous to the proton-motive force and cellular oxidative phosphorylation, prior to the existence of enzymes and complex lipid membranes.
b) Compare the energetic feasibility of this scenario with the "primordial soup" hypothesis in surface waters, considering the continuity of energy flux and protection against intense ultraviolet radiation and the Late Heavy Bombardment.
c) Predict what would happen to the redox potential and the organic synthesis capacity of this hydrothermal system if the external ocean lost its acidity and reached the same alkaline pH as the emerging fluids.
Analyse the following evolutionary information about mammals and dinosaurs: During the Mesozoic (about 140 Ma), mammals coexisted with dinosaurs while maintaining a…
Analyse the following evolutionary information about mammals and dinosaurs:
During the Mesozoic (about 140 Ma), mammals coexisted with dinosaurs while maintaining a reduced body size (under 5.0 kg), mostly nocturnal habits, and little varied dental morphology, despite already possessing endothermy and hair.
Non-avian dinosaurs went extinct at the K-Pg boundary (66 Ma ago). Over the following 15 Ma (Paleocene and Eocene), the fossil record shows a morphological explosion in mammals, with the sudden appearance of flying forms (bats), marine forms (first cetaceans), and large herbivores exceeding 1000 kg.
a) Explain this temporal contrast using the concepts of "vacant ecological niche" and "adaptive radiation". Why were the advantageous adaptations of mammals not sufficient on their own to trigger their diversification before the K-Pg boundary?
b) A classmate claims: "Catastrophes and mass extinctions are the primary engine that creates new species and accelerates the mutation rate". Reasonably refute this statement, clarifying what the actual biological role of mass extinctions in macroevolution is.
c) Reasonably predict which current animal group (for example, flightless birds, rodents, or arthropods) would have a higher probability of undergoing an equivalent adaptive radiation if all large terrestrial mammals disappeared today, and what selective pressures would guide their divergence.
Back to the beginning: the stromatolites of Cuatro Ciénegas are living relatives of the communities that, billions of years ago, filled the atmosphere with oxygen and paved the way for eukaryotic cells.
Summary
Let's practice! 3 medium exercises in this section