4,600 million years ago, Earth was a sphere of molten rock with not a single form of life. Today, from the jungles of Chiapas to the reefs of Cancún, there is an immense biological diversity. Between one moment and the other, matter went from nonliving to living, one of the great open problems in science. Here you will see the hypotheses that science proposes and the evidence that supports them. In Mexico, Alfonso L. Herrera (1868-1942) was a pioneer of these ideas with his "plasmogeny," which sought to imitate the origin of living matter in the laboratory.
Early Earth
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Describe what early Earth was like at the time life emerged: its atmosphere, its oceans and its geological activity.
Describe what early Earth was like at the time life emerged: its atmosphere, its oceans and its geological activity.
From spontaneous generation to prebiotic synthesis
With spontaneous generation ruled out, a question remained: how did the first life arise, then? In 1953, Stanley Miller and Harold Urey simulated the conditions of early Earth in the laboratory. In the diagram, the parts are labeled ?1, ?2, ?3, and ?4.
In the apparatus, the gas mixture (?1) imitates the early atmosphere, the electrical discharges (?2) imitate lightning, the boiling water (?3) imitates the oceans evaporating, and the trap (?4) collects what forms. The result: amino acids, the building blocks of proteins.
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Look at the diagram of the apparatus with which Miller and Urey carried out their famous experiment in 1953. Identify what the parts labelled ?1 to ?4 represent (the gas…
Look at the diagram of the apparatus with which Miller and Urey carried out their famous experiment in 1953. Identify what the parts labelled ?1 to ?4 represent (the gas mixture, electrical discharges, boiling water and the collection trap), explain what conditions of early Earth they simulated in the laboratory and what type of molecules they obtained.
For centuries, it was believed that living organisms could arise from non-living matter: worms from decaying meat or mice from dirty clothes. Describe what the…
For centuries, it was believed that living organisms could arise from non-living matter: worms from decaying meat or mice from dirty clothes. Describe what the hypothesis of spontaneous generation consisted of, and explain how it was experimentally disproved by Redi with his jars of meat, Spallanzani with boiled broths and, above all, Pasteur with his swan-neck flasks.
Current hypotheses on the origin of life include chemical synthesis in the "primordial soup", panspermia and hydrothermal vents on the ocean floor. Summarise them…
Current hypotheses on the origin of life include chemical synthesis in the "primordial soup", panspermia and hydrothermal vents on the ocean floor. Summarise them briefly and then order and explain the steps necessary to go from individual molecules to the first cell: appearance of hereditary material capable of copying itself, formation of complex organic molecules, appearance of metabolism and formation of membranes.
About $2\,400$ million years ago, the activity of cyanobacteria triggered the "Great Oxidation Event". Although today we consider oxygen an essential gas, for the…
About $2\,400$ million years ago, the activity of cyanobacteria triggered the "Great Oxidation Event". Although today we consider oxygen an essential gas, for the biosphere of that time it was a catastrophic event.
a) Justify why free oxygen ($O_2$) was lethal to the dominant microorganisms of that era and what key adaptive advantage was gained by organisms that developed aerobic cellular respiration.
b) Explain the mechanism by which the increase in $O_2$ reacted with atmospheric methane ($CH_4$) and triggered an almost total global glaciation (Huronian glaciation).
c) Deduce how the subsequent formation of the ozone layer ($O_3$) from this oxygen was a determining factor for the later colonisation of the continental surface by living organisms.
Stromatolites are layers of rock stacked like the pages of a book. Some are forming today: in Laguna Bacalar (Quintana Roo) and in Cuatro Ciénegas (Coahuila) there are colonies of microbes that trap sediment and precipitate minerals, building layered structures as they go. The fossils from 3,500 million years ago look like them. But is it enough for a rock to have layers to claim there was life?
The importance of cyanobacteria. If those structures on Earth are biological, they were built by cyanobacteria, the first organisms to carry out photosynthesis, releasing oxygen. Over millions of years of work, that production changed the atmosphere (the Great Oxidation Event), formed the ozone layer, and made life that breathes oxygen possible.
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An exploration rover on Mars photographs a sedimentary rock with finely layered and domed structures, visually identical to terrestrial stromatolites from $3\,500$…
An exploration rover on Mars photographs a sedimentary rock with finely layered and domed structures, visually identical to terrestrial stromatolites from $3\,500$ million years ago.
a) Justify why the presence of superimposed layers alone is not enough to confirm the existence of past life on Mars and what geochemical or isotopic analysis (for example, the ratio between $^{12}\text{C}$ and $^{13}\text{C}$) should be carried out to provide evidence for a biological origin rather than a purely inorganic process.
b) Explain the biological and environmental significance that stromatolite-forming cyanobacteria had in altering the atmosphere and the precipitation of iron in the oceans of early Earth.
For life to appear, liquid water and an atmosphere that protected and retained heat were needed. Earth has both because it is wrapped in two fluid layers.
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A space agency is planning a mission to detect potential metabolically active (non-fossil) microbial life forms in the Solar System and has the following data on three…
A space agency is planning a mission to detect potential metabolically active (non-fossil) microbial life forms in the Solar System and has the following data on three candidates:
Mars: Surface exposed to intense ultraviolet radiation and oxidizing perchlorates. Presence of seasonal methane (CH₄) peaks in the atmosphere and possible accumulations of liquid brines kilometers deep.
Europa: Global saline ocean beneath an ice sheet 15 to 25 km thick, in direct contact with a rocky seabed subjected to tidal forces from Jupiter's gravity.
Enceladus: Global subsurface ocean that expels continuous geysers into space through fractures at the south pole. Water vapor, salts, organic macromolecules, and dihydrogen gas (H₂) have been measured in these plumes.
Answer the following questions with reasoned arguments:
a) Using terrestrial mid-ocean ridge ecosystems as an analogous model, justify why the co-detection of H₂ and water in the geysers of Enceladus is considered a key indirect piece of evidence for habitability by chemosynthetic microorganisms.
b) Compare the biological and engineering difficulties of collecting viable samples on Europa versus Enceladus. Justify which of the two worlds offers a strategic advantage for a probe that can only perform orbital flybys without landing on the surface.
c) If the rover exploring Mars were to detect amino acids in the surface regolith, explain why this finding would not conclusively confirm the existence of past or present life, and what chemical property (chirality) scientists should analyze to obtain strong, though not definitive, evidence of whether their origin is biological or purely abiotic.
Astrobiologists look for liquid water before anything else. What does water have that no other substance has? Think about what happens in a swimming pool, in a lake that freezes over, or in the sweat that cools you on a hot day.
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Why is the search for life on other worlds so focused on finding liquid water? Explain which properties of water make it so important for living things.
Why is the search for life on other worlds so focused on finding liquid water? Explain which properties of water make it so important for living things.
Looking at the sky on the Night of the Stars and wondering whether we are alone is no longer just a philosophical question: space telescopes are discovering thousands of exoplanets and probes are analyzing jets of water on moons of Jupiter and Saturn. How can science know whether there is or was life on any of those worlds?
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What is astrobiology? Explain what it studies and why searching for life outside Earth helps us understand the origin of life on our planet.
What is astrobiology? Explain what it studies and why searching for life outside Earth helps us understand the origin of life on our planet.
Back to the beginning: the passage from molten rock to the biosphere required the planet to cool so it could have liquid water, organic molecules to form from nonliving matter, and those molecules to organize into structures with a membrane, metabolism, and hereditary material.
Summary
Let's practice! 3 medium exercises in this section