What was the first living organism?

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What was the first life form and how would you make the distinction or cutoff from what is inanimate and what is living? When do mixes of molecules stop being particles and start being alive?
 

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DaveC426913
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It is surely a fuzzy line between non-life and life, but there are some definitions that distinguish them. Here is one set of criteria:

  • Organization: being structurally composed of one or more cells
  • Homeostasis: regulation of the internal environment
  • Metabolism: transformation of energy by converting chemicals and energy into cellular components
  • Growth: a growing organism increases in size in all of its parts, rather than simply accumulating matter.
  • Response to stimuli: a response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
  • Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms.
 
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BillTre
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An approach to this kind of question that I like is to look at what features archaea and bacteria share in common. Archaea appear very similar to bacteria, but are a distinct and equally ancient group. Their shared features are likely inherited from their original common ancestor, while differences presumably arose later.
Common features of these two groups include:
  • having a membrane (but not its chemical composition)
  • the translation mechanism to make proteins from nucleic acid encoded sequences (ribosomes and tRNAs, however the ribosomes have some differences between the two groups)
  • the electron transport system which uses redox driven movement of electrons to pump H+ ions out of the cell
  • an ATPase which, driven by the different H+ concentration, makes ATP (the major chemical energy currency in cells)
  • RNA and DNA
  • some but not all DNA synthetic enzymes
Another common approach is to consider how life might have arisen on the early earth. The most reasonable scenario (IMHO, however there are several other hypotheses) makes use of natural geochemically driven situations were fluids with very different redox potentials come together generating large differences in redox potential. These can be found in deep sea alkaline hydrothermal vents (white smokers driven by the serpentinization of basalt rather than black smokers driven by the heat of recently cooled magma). The composition of post-Hadean oceans (different from today's oceans) are thought to have resulted in an even larger redox potential between the two fluids than is found now. By using iron, nickel, and sulfur compounds (also more abundant in the oceans at the time) as catalysts, redox reactions reduced CO2 and formed increasingly complex organic compounds, in some not fully understood manner.
Then membranes formed along with the electron transport system, the ATPase, metabolism, and nucleic acids.
Eventually these things were able to escape from the vents when they were able to become independent for their energy needs from their reliance on the unique redox situation they arose in.
It is sometime in along here that archaea and bacteria are thought to have split. Thus, their common ancestor would have been earlier (presumably defined as alive by then).

What would the first life form have been like?
Depends how you want to define it and on unknown details , but this kind of thinking can provide some constraints.
For Darwinian evolution to happen (cited as a criteria for life by some, but argued against by others) would require a nucleic acid encoding system for proteins and cell membranes to provide a keep everything together, house the electron transport system.
Keeping everything (molecular) together (with the membrane) would make biochemistry work better (products of synthesis not diffusing away). It would also be important for Darwinian evolution, in that it also provides a unique entity for selection to operate upon. Such a selected entity would contain a whole constellation of molecular components that could function together. It would also provide a barrier to genetic parasites that would try to take advantage of any successful organism.
 
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pinball1970
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jim mcnamara
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Thread locked - digression into speculation.
 

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