The Rise of the Cell
(Published in The EarthSphere Blog. Cover Image: First life — glowing chemosynthetic bacteria — cradled in the hand of existence (by WM House; ArcheanArt)
Last week I wrote about the origins of the building blocks of life: DNA and RNA. This article picks up the story and offers a few views on life’s jump from a non-living world to a living biosphere.
Abiogenesis as described by the Bible (Genesis 1): “God called the dry land Earth, and the waters that were gathered together he called Seas. And God saw that it was good. And God said, “Let the earth put forth vegetation, plants yielding seed, and fruit tree bearing fruit in which is their seed, each according to its kind, upon the earth.” (life arises from a lifeless Earth)
End of the Hadean Eon
As the Hadean Eon ended about four billion years ago, DNA or its precursors became established in Earth’s ancient oceans, and life was poised to take its first evolutionary step forward. Life needed to spontaneously arise from a lifeless ocean. Abiogenesis is the natural process of life emerging from non-living organic compounds. We talk about this process, but we don’t fully understand it. Gaps in our knowledge obscure the details of first life. But before we engage in a discussion about life, we require a framework for distinguishing when groups of molecules and chemical compounds pass from being non-living matter into a living organism.
There is no set definition for what constitutes life, and instead, we must look to the basic qualities differentiating flora and fauna from rocks. Most people broadly consider living organisms as complex, highly organized molecular assemblages with an ability to extract energy from their immediate environment and use it for growth and reproduction.
One of the basic concepts defining life is homeostasis, or the ability of an organized system to maintain itself in a steady state with its exterior environment. A single cell has an internal environment it must support. It must also have the ability to interact with conditions outside of its cell wall. As exterior conditions change, the cell monitors those changes and adjusts to maintain its internal environment. But homeostasis can also define certain non-living things like organizations or industrial complexes. Thus, we need other qualities to pin down life.
Viruses Dead or Alive
Achieving homeostasis requires work, and work means acquiring and expending energy. So we generally perceive living organisms as engaging in acquiring, processing, and metabolizing molecular compounds and other nutrients to generate energy. However, this requirement comes into question when we consider a virus with no ability to metabolize and generate energy for reproduction, but a clear ability to reproduce by utilizing the host cells it infects. Is it alive, dead, or some sort of zombie lifeform?
The lowly virus appears to be another species trying to survive like the rest of life in Earth’s biosphere. However, viruses occupy a fuzzy space on the edges of life. They are sometimes referred to as pseudo-living organisms because viruses don’t have a cellular structure. They are simple organisms, but they still haunt humanity in the form of viral pandemics like COVID-19.
Almost all plant and animal life we see around us is cellular. Some organisms are single-cell creatures, but others, like humans, are multi-cellular, containing trillions of cells. Whether an organism is single-celled or multi-celled, it reproduces by cellular division. Animal cells have the metabolic machinery to create new cells. Viruses lack any metabolism and must reproduce by hijacking the cellular machinery of another organism.
Yet, a virus has aspects of cellular life. It possesses genetic material, reproduces, and evolves by natural selection. In these respects, it seems alive. The basic physical components of a virus are nucleic acid (DNA or RNA) wrapped in a protein shell. Viruses have a history on Earth as long as cellular life. They arose in the primordial mix of first life, and regardless of whether we consider them alive, they remain a vital part of the biosphere four billion years later.
The Mighty Cell
Given our latitude in defining earliest life, it is worth considering the division of simple life into two main categories: viral and cellular. Cellular life further breaks into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea have an outer cell membrane, but their genetic material floats lose within the cell. The composition of their cell membranes differentiates the two. Eukarya also have an outer cell wall, but their genetic material is contained within a separate enclosed nucleus, nestled within the outer cell wall.
We don’t know which came first, viruses or cellular life. Some researchers posit they both evolved from a common pseudo-cellular life form — cellular biology moved towards increasing complexity, and viral morphology regressed into a simpler form.
Speculation about Earth’s earliest life forms requires we account for the biological pathways necessary to access energy either through creating it internally or capturing it from another organism. Today, most organisms rely on free oxygen to create energy. But free oxygen would have been a scarce commodity in Earth’s first oceans, so the earliest life forms must have consisted of anaerobic, chemosynthetic autotrophs. Chemosynthetic refers to the use of inorganic molecules as a metabolic energy source, and anaerobic indicates metabolic activity occurring in the absence of oxygen. When an organism uses its metabolic energy to produce its own carbon-based food, we call it an autotroph.
Life’s first challenge was achieving homeostasis by adapting to conditions in Earth’s oceans. Chemosynthetic bacteria were superbly suited to this task and they were probably the first cellular life forms, kicking off a grand evolutionary party.
The EarthSphere Blog: Exploring life and the planet supporting it.
More from ArcheanWeb:
ArcheanWeb: Exploring the environment, art, science, and more
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ArcheanWeb On Medium:
EarthSphere Publication — Science and the environment
Dropstone Publication — Stories, life observations, art, and more
Reflections on life’s journey and thoughts on the Tao Te Ching — In Search of a Path
A fictional adventure about the origins of life — The Strings of Life
Stories in progress on WattPad
Life’s Working Definition: Does It Work? (Source: NASA)
Simple Life Form May Have Existed 700 Million Years Earlier Than Previously Thought (Source: Science Daily)
The Two Empires and Three Domains of Life in the Postgenomic Age (by Eugene V. Koonin; Scitable)
The Origins of Viruses (by David R. Wessner; Scitable)
What came first, cells or viruses? (By Viviane Richter; Cosmos)