Exercises: The origin and evolution of life

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.

What are stromatolites and which organisms form them? Explain why they are considered one of the oldest pieces of evidence of life on Earth.

Compare the "primordial soup" hypothesis with the panspermia hypothesis: how they agree, how they differ, and what problem the second leaves unresolved.

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.

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 consequences of this change for life and for the Earth's surface.

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.

1. 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.
2. 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.

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⁺.

1. 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.
2. 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.
3. 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 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.

1. 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?
2. 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.
3. 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.