AP Syllabus focus:
‘Photosynthesis first evolved in prokaryotes; cyanobacterial photosynthesis oxygenated Earth’s atmosphere and underlies eukaryotic photosynthesis.’
Photosynthesis reshaped Earth by turning sunlight into chemical energy and transforming global chemistry. Understanding how it evolved—from early prokaryotes to chloroplast-bearing eukaryotes—explains major biological innovations and the rise of complex life.
Early Evolution: Photosynthesis Begins in Prokaryotes
Why prokaryotes were first
Early Earth lacked atmospheric oxygen, so the first photosynthetic organisms were prokaryotes that could harvest light energy in oxygen-poor environments. Natural selection favored variants that captured light more efficiently and used abundant electron sources.
Key evolutionary step: oxygenic photosynthesis
A major innovation was oxygenic photosynthesis, in which water serves as the electron donor, producing oxygen as a byproduct.
This diagram contrasts oxygenic photosynthesis (used by cyanobacteria and plants) with anoxygenic photosynthesis (used by other photosynthetic bacteria). It highlights the key evolutionary shift in electron donors: using yields as a byproduct, whereas using alternative donors (e.g., ) does not produce oxygen. The figure reinforces why oxygenic photosynthesis was a planetary “game changer.” Source
This pathway is strongly associated with cyanobacteria, whose adaptations allowed them to thrive in illuminated aquatic environments and spread widely.
Early photosynthetic strategies likely diversified among prokaryotes because:
Light is a consistent energy source at Earth’s surface.
Electron donors/acceptors varied by environment, promoting metabolic innovation.
Microbial populations evolve rapidly, enabling efficient refinement of energy-capturing systems.
Cyanobacteria and the Oxygenation of Earth
Oxygen as a planetary change agent
As cyanobacteria proliferated, their photosynthesis released increasing amounts of oxygen (O₂). Initially, oxygen reacted with dissolved iron and other reduced compounds, limiting atmospheric buildup. Over long timescales, these “oxygen sinks” became saturated, allowing oxygen to accumulate in the atmosphere.
Major consequences of rising oxygen included:
Redox shifts in oceans and atmosphere, changing which chemical reactions were energetically favorable
Selective pressure for oxygen-tolerant cells and, later, organisms that exploited oxygen in metabolism
Decline of many oxygen-sensitive anaerobes in oxygenated surface environments
Biological significance of atmospheric oxygen
Oxygen accumulation enabled new ecological opportunities and higher-energy lifestyles.
Oxygen’s significance for life includes:
Supporting aerobic metabolism, which yields much more usable energy per fuel molecule than anaerobic pathways
Promoting larger body size and more energy-demanding cellular functions over evolutionary time
Contributing to the formation of an ozone (O₃) layer, which reduces UV damage and helps organisms persist in surface and shallow-water habitats
From Prokaryotes to Eukaryotes: Photosynthesis Underlies Chloroplasts
Endosymbiotic origin of eukaryotic photosynthesis
Eukaryotic photosynthesis is explained by endosymbiosis, in which an ancestral eukaryotic cell took up a photosynthetic prokaryote and retained it as a long-term internal partner that became the chloroplast.
This figure depicts the endosymbiotic sequence leading to modern eukaryotes, including the later incorporation of a photosynthetic bacterium that became the chloroplast. The stepwise layout connects cellular engulfment to the emergence of double-membraned energy organelles. It visually reinforces why chloroplasts are considered derived from prokaryotes rather than invented de novo by eukaryotes. Source
The significance of this event is that it:
Connected prokaryotic light-harvesting chemistry to eukaryotic cell organisation
Enabled the diversification of photosynthetic eukaryotes (e.g., algae and plants)
Supported complex food webs by dramatically increasing global primary production (biomass generation from CO₂)
Endosymbiosis: A long-term symbiotic relationship in which one cell lives inside another, with the internal cell often evolving into an organelle-like structure over time.
Evidence Emphasised in AP Context
Multiple lines of evidence support the evolutionary storyline that prokaryotic photosynthesis came first, cyanobacteria oxygenated Earth, and eukaryotic photosynthesis derives from that prokaryotic foundation.
Evidence commonly cited includes:
Ancient microbial structures (e.g., stromatolite-like formations) associated with early photosynthetic communities
Geological signatures consistent with increasing environmental oxygen over time
Chloroplast features that resemble bacteria (e.g., their own DNA and bacterial-like components), consistent with a prokaryotic origin
FAQ
They look for converging signals: patterns in isotope ratios, minerals that form only under specific redox conditions, and sedimentary features that fit sustained, global oxygen production rather than local, abiotic events.
Early $O_2$ was rapidly consumed by “sinks” such as dissolved iron and reduced volcanic gases. Only after these sinks diminished could $O_2$ persist and build up in the atmosphere.
Yes. Oxygen is reactive and can damage cells. Many obligate anaerobes were pushed into anoxic refuges or went extinct, while oxygen-tolerant lineages diversified.
Commonly cited evidence includes chloroplast DNA, bacterial-like ribosomes, division resembling binary fission, and membrane traits consistent with an engulfment origin plus subsequent integration.
It drives most global primary production, supplying organic carbon and energy to food webs, and it maintains atmospheric $O_2$ levels that many organisms rely on for high-yield respiration.
Practice Questions
State why cyanobacterial photosynthesis is considered significant in Earth’s history. (2 marks)
Cyanobacteria carried out oxygenic photosynthesis releasing (1)
This oxygenated the atmosphere and enabled aerobic life / major ecological change (1)
Explain how photosynthesis evolved and why it was essential for the evolution of complex life. Your answer should include (i) where photosynthesis first evolved, (ii) the role of cyanobacteria in changing Earth’s atmosphere, and (iii) how eukaryotic photosynthesis arose. (6 marks)
Photosynthesis first evolved in prokaryotes (1)
Cyanobacteria evolved/used oxygenic photosynthesis using water, releasing (1)
Oxygen initially reacted with reduced materials, later accumulated in the atmosphere (1)
Rising oxygen enabled aerobic metabolism with higher energy yield (1)
Oxygen contributed to ozone formation reducing UV damage (1)
Eukaryotic photosynthesis arose via endosymbiosis leading to chloroplasts (1)
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