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  • Denis Pepin

Living Cells on Earth: Origins, Diversity, and Evolution

Updated: Mar 30

A man and a woman explore a surreal landscape teeming with diverse, colorful cells, illustrating the complexity and beauty of life at its most fundamental level.
This image depicts a journey through the microscopic world, where a man and a woman explore the diverse and complex forms of life that exist at the cellular level. The image illustrates the beauty and variety of living cells, as well as the common origin and evolution of all life forms on Earth. The image also invites the viewer to wonder about the mysteries and possibilities of life beyond our planet.
Introduction

Life is a complex phenomenon that emerges from the interactions of physical and chemical processes. The basic unit of life is the cell, a microscopic structure that can perform essential functions such as metabolism, growth, reproduction, and response to stimuli. Cells are composed of biomolecules, such as proteins, nucleic acids, lipids, and carbohydrates, that are synthesized from simpler molecules available in the environment.

The origin of life on Earth is one of the most intriguing and challenging questions in science. How did the first cells arise from non-living matter? What were the conditions and mechanisms that enabled the emergence and evolution of life? How did life diversify into the myriad forms that we observe today? These are some of the questions that scientists from various disciplines, such as biology, chemistry, geology, physics, and astronomy, try to answer using different methods and approaches.


In this article, we will explore some of the current hypotheses and evidence about the origin and evolution of life on Earth, and compare the number and diversity of living cells with the number and diversity of stars in the universe. We will also discuss some of the concepts and tools that are used to study life, such as planetology, geophysics, chemistry, biology, and fossils. We will focus on the role of cyanobacteria, oxygen, photosynthesis, and carbon dioxide in the history of life, and examine some of the major transitions and events that shaped the evolution of life, such as the origin of the first cell, the emergence of multicellularity, the colonization of land, and the rise of animals.



Living Cells on Earth: Number and Diversity

One way to estimate the number of living cells on Earth is to use the biomass, or the total mass of living organisms, as a proxy. Biomass can be measured in terms of carbon content, since carbon is a common element in all living matter. According to a recent study, the global biomass of living organisms is about 550 gigatons of carbon (Gt C), of which about 450 Gt C are in plants, 70 Gt C are in bacteria, 15 Gt C are in fungi, 2 Gt C are in animals, and the rest are in other groups, such as archaea, protists, and viruses.


Based on these estimates, the number of living cells on Earth can be calculated by dividing the biomass of each group by the average mass of a cell in that group. For example, the average mass of a bacterial cell is about 10^-12 grams, so the number of bacterial cells on Earth is about 70 Gt C / 10^-12 g = 7 x 10^29. Similarly, the average mass of a plant cell is about 10^-9 grams, so the number of plant cells on Earth is about 450 Gt C / 10^-9 g = 4.5 x 10^26. The average mass of an animal cell is about 10^-10 grams, so the number of animal cells on Earth is about 2 Gt C / 10^-10 g = 2 x 10^28.


Adding up the numbers of cells from all groups, we get a rough estimate of the total number of living cells on Earth, which is about 8 x 10^29. This is a very large number, but how does it compare to the number of stars in the universe?


The number of stars in the universe is not known precisely, but astronomers use various methods to estimate it based on observations and models. One method is to count the number of stars in a representative sample of galaxies, and then multiply it by the number of galaxies in the observable universe. Another method is to measure the luminosity, or the total amount of light emitted, of the observable universe, and then divide it by the average luminosity of a star. Both methods yield similar results, which suggest that the number of stars in the universe is about 10^23.


Therefore, the number of living cells on Earth is about 10^6 times larger than the number of stars in the universe. This means that for every star in the universe, there are about a million living cells on Earth. However, this does not mean that life is more abundant than stars in the universe, since stars are much larger and heavier than cells. In fact, the total mass of all stars in the universe is about 10^53 kg, while the total mass of all living cells on Earth is about 5.5 x 10^14 kg, which is about 10^38 times smaller.



The number of living cells on Earth reflects the diversity of life, which is the result of billions of years of evolution. Life on Earth is classified into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotes, which are single-celled organisms that lack a nucleus and other membrane-bound organelles. Eukarya are eukaryotes, which are organisms that have a nucleus and other membrane-bound organelles, and can be either single-celled or multicellular. Eukarya are further divided into four kingdoms: Protista, Fungi, Plantae, and Animalia.


Within each domain and kingdom, there are many phyla, classes, orders, families, genera, and species, which reflect the evolutionary relationships and similarities among living organisms. For example, animals belong to the kingdom Animalia, which is divided into about 35 phyla, such as Chordata (vertebrates and their relatives), Arthropoda (insects, spiders, crustaceans, and their relatives), Mollusca (snails, clams, squids, and their relatives), and Cnidaria (jellyfish, corals, anemones, and their relatives). Each phylum is further divided into classes, such as Mammalia, Aves, Reptilia, Amphibia, and Pisces within Chordata, and each class is further divided into orders, such as Primates, Carnivora, Rodentia, and Cetacea within Mammalia.


The diversity of life on Earth is estimated by the number of species, which are groups of organisms that can interbreed and produce fertile offspring. However, the exact definition and identification of species is not always clear, and different methods and criteria may yield different results. Therefore, the number of species on Earth is not known precisely, but it is estimated to be between 5 million and 30 million, of which about 1.8 million have been formally described and named.


The majority of known species are animals, which account for about 1.3 million species, followed by plants, which account for about 300,000 species, fungi, which account for about 100,000 species, and protists, which account for about 80,000 species. Bacteria and archaea are less well known, but they are estimated to have millions of species, many of which have not been discovered yet.


The diversity of life on Earth is not evenly distributed, but varies according to geographic regions, habitats, and environmental factors. Some regions and habitats have higher biodiversity than others, such as tropical rainforests, coral reefs, and islands, which harbor many endemic species, or species that are found only in a specific area. Biodiversity is also influenced by human activities, such as deforestation, pollution, overexploitation, and climate change, which can reduce or threaten the survival of many species.


Biodiversity is important for the functioning and stability of ecosystems, which are communities of living organisms and their physical environment. Ecosystems provide various services and benefits for humans and other organisms, such as food, water, oxygen, climate regulation, nutrient cycling, pollination, pest control, and recreation. Biodiversity also contributes to the aesthetic, cultural, and scientific value of life on Earth.



Origin of Life on Earth: Hypotheses and Evidence

The origin of life on Earth is a major scientific question that has fascinated and puzzled humans for centuries. How did the first living cells emerge from non-living matter? What were the conditions and mechanisms that enabled the emergence and evolution of life? When and where did life begin on Earth? These are some of the questions that scientists try to answer using various hypotheses and evidence.


There is no consensus or definitive answer to these questions, but there are several hypotheses and scenarios that have been proposed and tested by scientists. Some of the main hypotheses and scenarios are:


  • The primordial soup hypothesis, which suggests that life originated in a warm and dilute aqueous solution, or "soup", of organic molecules that were synthesized by abiotic processes, such as lightning, ultraviolet radiation, or volcanic activity. These organic molecules then underwent chemical reactions and self-organization, leading to the formation of more complex molecules, such as nucleic acids and proteins, and eventually the first cells.

  • The hydrothermal vent hypothesis, which suggests that life originated in the deep-sea hydrothermal vents, where hot water rich in minerals and gases emerges from the Earth’s crust. These vents provide a source of energy and chemical gradients that can drive the synthesis and concentration of organic molecules, and also protect them from harmful radiation and meteorite impacts. The organic molecules then formed membrane-bound structures, such as vesicles or protocells, that could exchange materials and information with the environment, and eventually evolved into the first cells.

  • The panspermia hypothesis, which suggests that life originated elsewhere in the universe, and was transported to Earth by comets, asteroids, or other celestial bodies. These bodies could carry organic molecules or even living cells that could survive the harsh conditions of space and the impact on Earth. Once on Earth, these extraterrestrial life forms could adapt and evolve to the local environment, and give rise to the diversity of life on Earth.

  • The RNA world hypothesis, which suggests that life originated from RNA, a type of nucleic acid that can store genetic information and catalyze chemical reactions. RNA molecules could have been synthesized by abiotic processes, and then self-replicated and diversified by mutation and recombination. RNA molecules could also interact with other molecules, such as lipids and proteins, and form more complex structures, such as ribosomes and viruses. RNA molecules could then give rise to DNA, a more stable and efficient type of nucleic acid, and the first cells.


These hypotheses and scenarios are not mutually exclusive, and could have occurred in combination or sequence. However, they are not proven, and face many challenges and gaps in explaining the origin of life. For example, how did the organic molecules become sufficiently concentrated and organized in the primordial soup or the hydrothermal vents? How did the extraterrestrial life forms survive the journey and the impact on Earth? How did the RNA molecules acquire the ability to self-replicate and catalyze reactions? How did the first cells acquire the essential components and functions, such as membranes, metabolism, and genetic code?



To test and evaluate these hypotheses and scenarios, scientists use various types of evidence, such as:


  • Experimental evidence, which involves recreating the conditions and processes that could have led to the origin of life in the laboratory, and observing the outcomes and products. For example, the famous Miller-Urey experiment, which simulated the early Earth’s atmosphere and ocean, and produced amino acids, the building blocks of proteins, by applying electric sparks.

  • Geological evidence, which involves studying the rocks and minerals that could have provided the environment and materials for the origin of life on Earth, and dating them using radioactive isotopes. For example, the oldest known rocks on Earth, which are about 4 billion years old, and contain traces of carbon isotopes that suggest the presence of organic matter.

  • Fossil evidence, which involves studying the preserved remains or impressions of ancient organisms that could indicate the earliest forms of life on Earth, and dating them using radioactive isotopes or stratigraphy. For example, the oldest known fossils on Earth, which are about 3.5 billion years old, and consist of stromatolites, which are layered structures formed by cyanobacteria, a type of photosynthetic bacteria.

  • Molecular evidence, which involves studying the biomolecules and genomes of living organisms that could reveal the evolutionary relationships and similarities among them, and estimating the divergence times using molecular clocks. For example, the universal phylogenetic tree, which shows the common ancestry and divergence of the three domains of life, and suggests that the last universal common ancestor (LUCA) existed about 3.8 billion years ago.


These types of evidence are not conclusive, and have limitations and uncertainties. For example, experimental evidence may not reflect the actual conditions and processes that occurred on the early Earth, and may be influenced by contamination or human intervention. Geological evidence may be scarce, incomplete, or altered by erosion, metamorphism, or plate tectonics. Fossil evidence may be rare, ambiguous, or misinterpreted, and may not represent the true diversity and complexity of ancient life. Molecular evidence may be affected by horizontal gene transfer, gene loss, or mutation rate variation, and may not account for the extinct or undiscovered lineages of life.


Therefore, the origin of life on Earth remains an open and active area of research, and requires interdisciplinary collaboration and integration of multiple sources and types of evidence.


Evolution of Life on Earth: Major Transitions and Events

The origin of life on Earth was not the end of the story, but the beginning of a long and complex process of evolution. Evolution is the change in the characteristics and diversity of living organisms over time, driven by natural selection, genetic drift, mutation, recombination, and other mechanisms. Evolution involves both microevolution, which is the change in the frequency of alleles and traits within a population or species, and macroevolution, which is the change in the number and diversity of species and higher taxa over geological time.


The evolution of life on Earth has been marked by several major transitions and events that have shaped the history and diversity of life. Some of these transitions and events are:


  • The origin of the first cell, which was the first self-replicating and metabolizing entity that could be considered alive. The first cell is thought to have been a prokaryote, a simple cell that lacks a nucleus and other membrane-bound organelles. The first cell may have originated from a protocell, a membrane-bound structure that contains organic molecules and can perform some basic functions, such as growth, division, and response to stimuli. The first cell may have also acquired the genetic code, a system that translates nucleic acid sequences into protein sequences, and the universal ancestor of all living organisms. The origin of the first cell is estimated to have occurred about 3.8 billion years ago, based on molecular and fossil evidence.

  • The origin of photosynthesis, which was the process that enabled some cells to capture light energy and convert it into chemical energy, and also produce oxygen as a by-product. Photosynthesis is one of the most important innovations in the history of life, as it provided a new source of energy and carbon for living organisms, and also changed the composition and chemistry of the atmosphere and the ocean. Photosynthesis is thought to have originated in cyanobacteria, a type of prokaryote that can perform oxygenic photosynthesis, which uses water as an electron donor and produces oxygen. Cyanobacteria are also responsible for forming stromatolites, the oldest known fossils on Earth. The origin of photosynthesis is estimated to have occurred about 3.5 billion years ago, based on molecular and fossil evidence.

  • The origin of eukaryotes, which were the cells that had a nucleus and other membrane-bound organelles, such as mitochondria and chloroplasts. Eukaryotes are more complex and diverse than prokaryotes, and can be either single-celled or multicellular. Eukaryotes are thought to have originated from the endosymbiosis of prokaryotes, a process that involves one cell engulfing another cell and forming a symbiotic relationship. The endosymbiosis of an aerobic bacterium gave rise to mitochondria, which are the organelles that produce energy by cellular respiration. The endosymbiosis of a cyanobacterium gave rise to chloroplasts, which are the organelles that perform photosynthesis. The origin of eukaryotes is estimated to have occurred about 2 billion years ago, based on molecular and fossil evidence.

  • The origin of multicellularity, which was the state of being composed of more than one cell that are connected and coordinated. Multicellularity is a key feature of many eukaryotes, and allows for the differentiation and specialization of cells, tissues, organs, and systems. Multicellularity also enables the emergence of complex structures and functions, such as movement, sensation, communication, and reproduction. Multicellularity is thought to have originated independently in several lineages of eukaryotes, such as plants, fungi, and animals, by various mechanisms, such as aggregation, division, or fusion of cells. The origin of multicellularity is estimated to have occurred about 1.5 billion years ago, based on molecular and fossil evidence.



  • The colonization of land, which was the expansion of life from the aquatic to the terrestrial environment. Land colonization was a major challenge and opportunity for living organisms, as it involved adapting to new conditions, such as gravity, desiccation, temperature, and light. Land colonization also involved exploiting new resources, such as soil, minerals, and air. Land colonization is thought to have occurred in several groups of eukaryotes, such as plants, fungi, and animals, by various adaptations, such as cuticles, stomata, spores, seeds, roots, stems, leaves, lungs, legs, and wings. The colonization of land is estimated to have occurred about 500 million years ago, based on molecular and fossil evidence.

  • The rise of animals, which were the multicellular eukaryotes that belong to the kingdom Animalia, and have characteristics such as heterotrophy, motility, nervous system, and sexual reproduction. Animals are the most diverse and abundant group of living organisms on Earth, and occupy almost every habitat and niche. Animals are thought to have originated from a single-celled ancestor that was related to choanoflagellates, a type of protist that has a flagellum and a collar of microvilli. Animals are also thought to have diversified rapidly during the Cambrian explosion, a period of about 50 million years that saw the emergence of most of the major animal phyla and body plans. The origin of animals is estimated to have occurred about 800 million years ago, based on molecular and fossil evidence. The Cambrian explosion is estimated to have occurred about 540 million years ago, based on fossil evidence.


These transitions and events are not exhaustive, and there are many other important and interesting aspects and episodes in the evolution of life on Earth, such as the origin of sex, the origin of plants, the origin of fungi, the origin of vertebrates, the origin of mammals, the origin of humans, and the mass extinctions and radiations that have affected the diversity and distribution of life. The evolution of life on Earth is a continuous and ongoing process that is influenced by various factors, such as genetic variation, natural selection, environmental change, and biotic interactions. The evolution of life on Earth is also a fascinating and complex story that reveals the origin, history, and diversity of life.


Conclusion


Life on Earth is a remarkable phenomenon that has emerged and evolved over billions of years, from the first cell to the millions of species that exist today. Life on Earth is also a diverse and complex phenomenon that can be studied and understood from various perspectives and disciplines, such as planetology, geophysics, chemistry, biology, and fossils. Life on Earth is also a dynamic and ongoing phenomenon that is influenced and challenged by various factors and events, such as natural selection, environmental change, and mass extinctions.


In this article, we have explored some of the current hypotheses and evidence about the origin and evolution of life on Earth, and compared the number and diversity of living cells with the number and diversity of stars in the universe. We have also discussed some of the concepts and tools that are used to study life, such as biomass, species, and phylogenetic tree. We have also focused on the role of cyanobacteria, oxygen, photosynthesis, and carbon dioxide in the history of life, and examined some of the major transitions and events that shaped the evolution of life, such as the origin of the first cell, the emergence of multicellularity, the colonization of land, and the rise of animals.


However, this article is not comprehensive, and there are many other aspects and episodes in the evolution of life on Earth that are not covered here, such as the origin of sex, the origin of plants, the origin of fungi, the origin of vertebrates, the origin of mammals, the origin of humans, and the mass extinctions and radiations that have affected the diversity and distribution of life. The evolution of life on Earth is a vast and fascinating topic that invites further exploration and discovery.


I hope you enjoyed reading this article, and learned something new and interesting about life on Earth. Thank you for your attention and interest.


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