Self-contained, integrated structures {life}| have mechanisms for gathering and using energy and matter, to build mechanisms and reproduce similar structures. Living organism species come from one or two genetically similar organisms. Species communities live in local regions. Community ecosystems live in large geographic regions or climate zones. Living things adapt, grow, have irritability, and reproduce. Living things have sizes, shapes, biochemical reactions, molecules, and movements.
Biochemical molecules, organelle molecular structures, cell molecular systems, tissue cell types, and organ-system tissue groups can work as units {organism}| {individual}. Organisms eat each other, live in different environments, and use oxygen differently.
Killing can be for the sake of mercy {euthanasia}|, either by letting people die or by painless killing.
Organisms can react to stimuli {irritability, organism}|.
Perhaps, living things need special energy {life force} for motion and organization. However, molecules and physical laws can make life. Organic molecules, cells, organisms, species, and all life can be purely physical and require no extra information or non-physical energy.
Perhaps, living things can arise directly from molecules or decaying matter {spontaneous generation}|. However, organisms are too complex to arise directly from molecules or decaying matter. Organisms arise only from other living things, which contain information needed to initiate and develop life through complex processes. At life's origin, more-complex molecules, between living and non-living, arose from complex molecules by moderately complex processes.
head {cephalic}.
about head {cranial}.
about body fluids {humoral}.
middle {medial}|.
back {dorsal}.
abdominal, anterior, or lower {ventral}|.
tail {caudal}.
head {rostral}|.
side {lateral, side}|.
opposite side {contralateral}|.
same side {ipsilateral}|.
Experiments simulating primitive Earth conditions can make small organic molecules. Under special conditions, these molecules can make large stable proteins, ribonucleic acids (RNA), and deoxyribonucleic acids (DNA) {origin of life}|. DNA has optimum mutation rate, crossing-over, hybridization, and long length and so can be replication templates.
early-Earth molecules
Experiments replicating early-Earth conditions make formaldehyde, formic acid, lactic acid, acetic acid, urea, sugars, and hydrogen cyanide. From nitrogen, methane, ammonia, water, and hydrogen-gas mixtures, lightning or ultraviolet light can produce amino acids. Metallic carbides and water can react to form acetylene. Formaldehyde can polymerize to make ribose and other sugars.
replication
Living things replicate, so life requires replicating molecules. Proteins cannot be templates, because most amino acids can have no hydrogen bonding. RNAs are easy to create. However, RNA is also easy to hydrolyze, so only short RNA regions can replicate. DNA does not hydrolyze, because deoxyribonucleotides have no oxygen atom and prevent hydrolysis. DNAs are harder to create but can be templates.
cell functions
After DNA formation, DNA regions able to make functional RNAs and proteins arose. To these exons, evolution added and subtracted introns. Archaebacteria have tRNA and rRNA introns. Cyanobacteria eubacteria have leucine-tRNA introns. Eukaryote RNAs typically have introns. Eukaryote DNA has different intron types, such as self-splicing introns. Currently, gene exons have 1000 to 7000 functional DNA regions.
cell functions: photosynthesis
Earth life needs photosynthesis, using metals, enzymes, carbon dioxide, and water.
chirality
Perhaps, chirality is necessary.
sexual reproduction
Earth life needs sexual reproduction, for more variation and more competition. Sexual reproduction began 2.2 x 10^9 years ago. Sexual intercourse began 2 x 10^8 years ago.
mutation
Different DNA types change at different rates. Mitochondrial DNA mutates ten times faster than nuclear DNA. Mitochondrial DNA mostly comes from mother, but some paternal genes can enter and recombine. DNA-change rates can be faster if codon changes do not change amino acids produced. DNA-change rate is slower for histones and other fundamental proteins. DNA-change rate is slower in humans than in other species.
gene duplication
With sexual reproduction, genes can duplicate by unequal crossing over at recombination. If genes duplicate, one copy can change while the other still provides original functions, thus allowing genetic drift.
complexity
DNA amount and gene number can increase for whole genome, tRNA, rRNA, mitochondrial DNA, and globin DNA, increasing organism complexity.
Percentage of planets that can have intelligent life depends on star formation rate, fraction of stars that have planets, percentage of planets that are suitable for life, fraction where life actually exists, intelligent-life probability, and average civilization longevity {Drake equation} (Frank Drake) [1961]. For planets to have life, they must be like Earth and have stars like Sun.
star
Sun is a yellow-orange class-G0 star. Only class F, G, and K stars can have suitable planets, because liquid-water zone is too small for smaller stars, and bigger stars have no rocky planets in that zone. Such stars have masses 0.7 to 1.5 times Sun mass. Lifetimes are long enough, and masses are big enough, for planets. Stars sufficiently like Sun are 1% of all stars.
Stars younger than Sun have time too short for life. Stars older have too few heavy elements. Time range is 3 to 7 billion years, one-third of all stars.
Multiple stars can have no planets. Single stars are one-fifth of all stars.
Stars with slow spins possibly indicate planets. Probably, one sixth of all stars qualify.
Stars must be in galactic arms. Galaxy centers have too much radiation. Edges have low metals and low star-formation rates. Galactic habitable zone is far from center and edge.
orbit
Circular orbits make temperature swings not too great. Probably 100% of planets at correct distance from star have circular orbits.
size
Earth size is big enough to retain oxygen and nitrogen and small enough to lose hydrogen and helium, so as not to have too much gas. Surface gravity is not too strong or too weak for living things. Diameter is 5000 km to 15000 km. Probably, one tenth of all planets have correct size. Therefore, 0.001% of all stars have Earth-like planets. Galaxy has 2.5 x 10^11 stars and so 2.5 x 10^6 habitable planets.
rotation
Planet rotation must not be too fast or slow. Probably 100% of planets at correct distance from star have Earth-like rotation speeds.
atmosphere
On early Earth, volcanoes gave off steam, nitrogen, methane, hydrogen cyanide, ammonia, carbon dioxide, and sulfur dioxide or hydrogen sulfide. If iron was already at core, atmosphere was carbon dioxide, nitrogen, and sulfur dioxide. Soon after Earth formed, atmosphere layered into decreasing-density gases. Ultraviolet light reaching Earth decreased, and temperature lowered. Crust cooled quickly, and lower temperature led to more atmosphere layering. Hydrogen, ammonia, and methane were no longer in oceans, so processes no longer formed organic molecules. Temperature became too low to make organic molecules. All gases can dissolve in oceans.
temperature
If planet surface temperature is hotter than 40 C, proteins denature and water evaporation is too high. If surface temperature is colder than ice, no water is available. Planets must be in circumstellar habitable zones. If planet forms too close to star, it has little water and large greenhouse effect, like Venus. If planet forms too far from star, surface is ice. Distance from star is 10^7 km for optimum temperature. Probably, one tenth of planetary systems have such planets. Composition, size, wind, rain, and sunlight cause tectonic and erosion processes.
minerals
If planet is at correct distance from star, mineral composition is similar to Earth mineral composition.
radiation
Cosmic radiation can react water and carbon dioxide to make organic acids.
energy
3,800,000,000 years ago, ultraviolet light, lightning, meteor impacts, thunder, volcanic heat, and hydrothermal vents provided energy.
meteors and comets
Perhaps, some organic molecules came to Earth in meteors and comets. Meteors have saturated hydrocarbons, porphyrin rings, and organic acids. Spores cannot come from space, because ultraviolet and ionizing radiation kill them.
ocean
First life probably arose in shallow seas or tidal areas. Oceans probably had water, gases, proteins, nucleic acids, carbohydrates, fats, and adenosine phosphate.
tides
Shallow seas with high evaporation rates allow molecule concentration. Tides add water. Large moons can cause tides. On Earth, tides were 30-meters high when Moon formed.
other factors
Probably, Earth life needs continental drift, orbital changes, star evolution, seasons, days and nights, major climactic changes, and magnetic fields.
catastrophe
Earth life needs no life-ending catastrophes, like too many comets or meteors, too much volcanism and earthquakes, too much erosion, or too much greenhouse effect.
life factors
Earth life needs brains, hands, vocal chords, speech centers, forebrains, vision, immune systems, and societies. Earth life must be large enough, be long-lived enough, be few in number, have slow reproduction cycles, and have long childhoods. Earth life must have no mass destruction, optimum competition, optimum population, enough energy and resources, few radioactive wastes, few chemical wastes, optimum ozone, and social cohesion.
number of times
On Earth-like planets, life has probability, possibly 10^-6. Perhaps, galaxies have 10^6 suitable planets. Therefore, galaxies have one planet with life. If other planets have intelligent life, they can send probes to Earth, but there is no evidence for this. Therefore, no other intelligent life forms are in Milky Way Galaxy yet.
first cells
Life began as non-photosynthetic one-celled bacteria-like organisms. First cells reproduced themselves, protected themselves, and found energy.
membrane
All cells have cell membranes. Cell membranes have lipids with embedded proteins. Cells can control membrane-molecule amounts and ratios. All cells have voltage differences across cell membranes, because inner and outer sodium, potassium, and chloride salt concentrations differ.
energy and entropy
Life can overcome dissipative forces and persist. Living systems have high order {negentropy} in small regions, surrounded by large energy sources that they can tap. Living systems must gather energy faster than surroundings can dissipate energy. Sunlight energy, planet interior heat, and lightning can make locally high temperature, for anabolic and catabolic chemical reactions.
Small cells have small fast energy changes. Fast heat and material exchanges among physical compartments allow rapidly removing order from surroundings. High combination and division rates make many new organic molecules and cells. Equilibrium conditions in oceans or tide pools allow reversible chemical reactions.
If life is abundant in universe, why have people not seen other intelligent life {Fermi paradox}?
Planets must have liquid water, not ice or steam {circumstellar habitable zone} (CHZ).
Galaxy centers have too much radiation. Galaxy edges have low metals and low star-formation rates. Stars must be in galactic arms {galactic habitable zone} (GHZ).
Hot water rich in sulfur, iron, hydrogen, and carbon flows from sea floor holes {hydrothermal vent}|. Bacteria-like organisms began there.
If surface had iron 3,800,000,000 years ago, reduction reactions caused methane, ammonia, and hydrogen sulfide {reducing atmosphere}.
Carbon compounds {carbon-based life} can be soluble or insoluble and have intermediate-stability chemical bonds, so they can form and break at moderate energies.
organic molecules
Organic molecules formed in seas. Stable forms persisted. Persisting molecules aggregated.
nucleic acids
When nucleotide concentrations were great enough, nucleotides linked. Nucleic acids used themselves as templates to make copies. Good copiers persisted and used more nucleotides. Mutations resulted in populations with property ranges.
peptides
When amino-acid concentrations were great enough, amino acids linked. Peptides assisted chemical reactions and formed structures.
oxygen
Later, photosynthesis added oxygen to atmosphere, using water, sunlight, and atmosphere carbon dioxide.
Silicon compounds {silicon-based life} cannot substitute for carbon compounds, because silicon compounds are insoluble, silicon makes shorter bonds, and silicates are too stable.
In living organisms, sugars and amino acids are in only one of two possible stereochemical forms {chirality, molecule}|. Sugar and amino-acid chirality probably had survival value, but people do not yet know cause [Gardner, 1960].
Perhaps, living things or organic molecules are in cosmic dust, meteors, comets, asteroids, and planets and then travel to Earth in dust, meteors, or comets {panspermia}.
Glycerin molecules mixed with other molecules can clump together to make stable gels {coascervate}| [1926]. Other molecules can enter, interact inside, and leave.
Nucleic acid and protein can combine. Perhaps, dissolved genes in early oceans evolved to make genes with protein shells {proto-virus}. Proto-viruses can acquire lipid layers, making micelles. Proteins can embed in lipids, to make cell membranes. Food can pass from seawater into simple cells. Chemosynthesis can evolve. Simple cells can replicate. Later, photosynthesis can evolve.
Perhaps, RNAs had genetic codes, were enzymes, and formed structures {RNA world}. RNAs folded and unfolded to perform functions.
Eukaryote-cell organelles can come from eubacteria incorporated into cells {endosymbiosis theory}. Plant chloroplasts came from cyanobacteria. Animal mitochondria came from purple photosynthetic bacteria. For example, Cryptomonas phytoflagellate combines eukaryotic nucleus, photosynthetic prokaryote, and eukaryotic cell.
Evolution has added introns to, and subtracted introns from, DNA, possibly to aid exon recombination {exon shuffling} and combine protein functions.
Bacteria have one circular chromosome {monoploid}|. Polyploid protozoa have several linear-chromosome copies. Perhaps, bacteria are simplified polyploid protozoa.
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Date Modified: 2022.0225