Human energy system
Chapter 2 - Society
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Welcome to the Human energy system page
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Key takeaways
- Energy is not just a background condition for evolution; it is a central driver that shapes the possibilities and limits of life
- By powering biological functions, enabling greater complexity, and serving as the currency of evolutionary fitness, energy flow underpins the entire evolutionary process
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Core ideas
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Energy is a prerequisite for life
- Without energy, biological processes such as growth, reproduction, and adaptation cannot occur. Thus, "no energy; no evolution".
Energy is a prerequisite for complexity
- The flow of energy through living systems enables the accumulation of biological information in molecules like DNA and RNA. This information, shaped by natural selection, leads to an increase in complexity over evolutionary time. Major evolutionary transitions, such as the emergence of eukaryotic cells, were made possible by innovations in energy acquisition and utilisation (e.g., the symbiosis that led to the development of mitochondria, which enabled higher energy yields and greater genetic complexity).
Energy supports evolutionary fitness
- The concept of "energetic fitness" quantifies how organisms convert biomass and energy into surviving offspring, and natural selection tends to maximise this efficiency. Organisms compete for energy resources. Those that acquire and use energy more efficiently have a selective advantage, leading to the evolution of traits that improve energy acquisition, storage, and use.
Energy supports the biosphere
- The efficiency of energy transfer and use at each trophic level influences the structure and dynamics of ecosystems, thus impacting evolutionary trajectories. The sun is the primary source of energy for life on Earth. Plants convert solar energy into chemical energy via photosynthesis, which then flows through food webs to support all other life forms.
Energy supports adaptation
- Within species, variations in mitochondrial genes can adjust energy metabolism to suit local environments, thereby supporting survival and reproduction under diverse conditions. Evolutionary adaptation often involves changes in how organisms exploit available energy sources. For example, mutations in nuclear DNA can enable species to access new energy reservoirs, leading to speciation.
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Dive deeper
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The role of energy in the emergence of biology
The evolution of bioenergetics might have been driven by invasion of new habitats by the primordial life forms. The proposed origin of the first cells at anoxic geothermal fields implies that life started not as a planetary but as a local phenomenon. The invasion of marine environments by membrane-encased organisms transformed life into a planetary phenomenon but at the same time challenged the cells by high sodium levels which could drive the emergence of membrane bioenergetics.
| The Role of Energy in the Emergence of Biology from Chemistry - Daria V Dibrova - 2014 |
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| Any scenario of the transition from chemistry to biology should include an “energy module” because life can exist only when supported by energy flow(s). |
| https://pmc.ncbi.nlm.nih.gov/articles/PMC3974900/ |
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Evolution
The ‘modern synthesis’ of the past century explained evolution in terms of genes, but this is only part of the story. While the mechanisms of natural selection are correct, and increasingly well understood, they do little to explain the actual trajectories taken by life on Earth.
| Energy, genes and evolution: introduction to an evolutionary synthesis - Nick Lane - 2013 |
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| Life is the harnessing of chemical energy in such a way that the energy-harnessing device makes a copy of itself. No energy, no evolution. |
| https://royalsocietypublishing.org/doi/10.1098/rstb.2012.0253 |
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Complexity
In a thermodynamically isolated system, complex structures decay toward randomness. However, in nonequilibrium systems, the flow of energy through the system generates and sustains structural complexity, and nonhomogeneous structures embody information. The flow of energy through biological structures permits them to reproduce, thus duplicating their DNA. Today, the primary energy source for terrestrial life is the flux of high-energy photons from the Sun through the biosphere.
| Bioenergetics, the origins of complexity, and the ascent of man - Douglas C Wallace - 2010 |
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| Complex structures are generated and maintained through energy flux. Structures embody information, and biological information is stored in nucleic acids. The progressive increase in biological complexity over geologic time is thus the consequence of the information-generating power of energy flow plus the information-accumulating capacity of DNA, winnowed by natural selection. |
| https://www.pnas.org/doi/full/10.1073/pnas.0914635107 |
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Energy & human evolution
"... the available energy per unit of time (available power) placed at the disposal of the organisms, for application to their life tasks and contests, may be capable of increase by increasing the rate of turnover of the organic matter through the lifecycle. ... Once more it is evident that, whenever a group of organisms arises which is so constituted as to increase the rate of circulation of matter through the systemin, natural selection will operate to preserve and increase such a group, provided always that there is presented a residue of untapped available energy-, and, where circumstances, require it, also a residue of mass suitable for the composition of living matter.
In every instance considered, natural selection will so operate as to
- increase the total mass of the organic system,
- to increase the rate of circulation of matter through the system,
- and to increase the total energy flux through the system,
so long as there is presented an un-utilized residue of matter and available energy.This may be expressed by saying that natural selection tends to make the energy flux through the system a maximum, so far as compatible with the constraints to which the system is subject."
| Contribution to the Energetics of Evolution - Alfred J. Lotka - PNAS - 1922 |
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| It has been pointed out by Boltzmann' that the fundamental object of contention in the life-struggle, in the evolution of the organic world, is available energy. In accord with this observation is the principle that,in the struiggle for existence, the advantage must go to those organisms whose energy-capturing devices are most efficient in directing available energy into channels favorable to the preservation of the species. |
| https://www.pnas.org/doi/10.1073/pnas.8.6.147 |
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Primate metabaolism
"Primates’ slow life history schedules are generally thought to reflect an evolved strategy of allocating energy away from growth and reproduction and toward somatic investment, particularly to the development and maintenance of large brains. We found that primates use remarkably little energy each day, expending on average only 50% of the energy expected for a placental mammal of similar mass."
| Primate energy expenditure and life history - Herman Pontzer - PNAS - 2024 |
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| Measurements of daily energy expenditure indicate that primates, including humans, expend only half of the calories expected for mammals of similar body size. As energy expenditure is central to organismal biology, these results hold important implications for life history, evolutionary biology, and foraging ecology for primates and other mammals. Specifically, we show that primates’ remarkably low metabolic rates account for their distinctively slow rates of growth, reproduction, and aging. |
| https://www.pnas.org/doi/10.1073/pnas.1316940111 |
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Human metabolism
"Metabolic quotients add to previous evidence that innovations in food procurement, food processing, and cooperative behaviors increase energy availability, thus permitting mass-specific energy expenditure in humans to be higher relative to our closest ape relatives. While wild chimpanzees walk on average 3 to 4 km/d, climb 100 m/d, and rarely sprint for only short distances, average hunter-gatherer females and males walk 9 and 15 km/d, respectively, often carrying heavy burdens, engage in other physical activities such as digging, and sometimes long-distance running."
| Metabolic scaling, energy allocation tradeoffs, and the evolution of humans’ unique metabolism - Andrew K. Yegian - PNAS - 2024 |
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| ... metabolic quotients to several human populations reveal that humans evolved exceptionally high metabolic rates that, unlike other mammals including nonhuman primates, do not trade off energy allocation to maintenance versus physical activity. Humans’ uniquely high metabolic rates helped fuel the evolution of our species’ large brains, high reproductive rates, and extended longevity.
All organisms use limited energy to grow, survive, and reproduce, necessitating energy allocation tradeoffs, but there is debate over how selection impacted metabolic budgets and tradeoffs in primates, including humans. ... data from several small-scale societies show that humans evolved exceptionally high resting, activity, and total metabolic rates apparently by overcoming tradeoffs between resting and active energy expenditures that constrain other primates. Enhanced metabolic rates help humans fuel expanded brains, faster reproductive rates, extended longevity, and high percentage of body fat. |
| https://www.pnas.org/doi/10.1073/pnas.2409674121 |
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