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Science can lead us to marvel at the uniqueness of our planet. Richard Hammond (Hammster) told us that 93% of Earth consists of iron, silicon, oxygen and magnesium (Fe, Si, O and Mg) ref: bbc.
With the addition of water (H₂O) in its liquid state, and atmospheric N₂ and CO₂, there came to be conditions in which living cells could appear. (Atoms of C, H, O and N make up the greater part of organic molecules.)
Water is the only known substance in which many molecules undergo reactions and can become 'organized' in a way that supports life. Even those organic molecules which are themselves not entirely water-soluble are seen to be organized by the presence of water. (For example, the long 'tails' of cell membrane molecules are hydrophobic: The membrane has a bi-layer structure and water on each side of it forces the hydrophobic 'tails' together internally, see: phospholipid.)
Volcanoes are essential for life because they emit CO₂ into our atmosphere. The carbon in the air holds onto some of the heat that's received from the sun. As a result, Earth's average ambient temperature has been high enough to ensure that much water on Earth has been in its liquid phase for a good part of the latest geological eon (see brit.) Venus has had much more volcanic activity in the past, and her atmosphere retains too much heat for any water to be present. Mars has white precipitation at the poles, but it is frozen CO₂.
Mostly, the Earth's surface temperatures remain above -30°C and below 50°C. Only three-hundred miles away, in space, we know that temperatures drop below -270°C. (At the other extreme, men have been able to generate a heat of 4 trillion °C inside the Large Hadron Collider. The record was beaten recently: hottest.) Where but on Earth are we likely to find such a narrow natural range of temperatures that there is always lots of liquid water around (not forgetting the effect of atmospheric pressure on the freezing and boiling points of water)?
For a long time, it was expected that all water in other parts of our solar system was in its solid phase. However, in 2005 it was discovered that water vapour erupts in plumes on the surface of one of Venus' icy moons, Enceladus. A sub-surface ocean is said to exist because Saturn's gravitational forces distort Enceladus in a tidal way, causing frictional heat that melts some of her near-surface ice. (This discovery of the liquid water has tempted some to suggest that life might appear in a way that it appeared at geothermal vents in Earth's oceans.)
Water has remarkable set of properties and it's been called the "universal solvent" that makes life possible. The angle of 104.5° in its H-O-H structure confers a polarity that makes it such a good solvent.
Water reaches its maximum density at 4°C and that's why ice floats. This is vital to aquatic life because a floating sheet of ice provides thermal insulation: Large bodies of water do not freeze solid and sea creatures stay warm enough under a frozen surface. Freshwater organisms also survive under a surface sheet of ice.
As well as being the only known planet with so much liquid water, Earth has a unique coincidence of features, without which life is very unlikely to develop. For example:
- Earth spins and its rotation is stabilized by a non-rotating moon, ensuring that most of Earth's surface gets some sunshine all year. (Only the poles are in darkness is winter>)
- Like a 'thermal blanket', our atmosphere makes almost all places inhabitable on Earth, even when there is no sunlight (i.e. in winter at the poles.) Heat is retained in the atmosphere by carbon that is belched from volcanoes. Therefore, air is warm enough for us to breathe, even in the coldest regions. In contrast on our moon, it is 273°C colder in the 'night'. (The Earth's atmosphere also shields us from more than one form of solar radiation that is harmful to life.)
- Earth has a molten core that spins and creates a magnetic field, which shields our atmosphere from solar winds. (Earth's smaller 'sister', Mars, has lost most of her atmosphere because her core is much less active now and it generates a weaker magnetic shield.)
- Earth's axial tilt gives rise to seasons while orbiting the sun: Warm seasons loosen up frozen ocean surfaces at the poles, and currents then move nutrients to support phytoplankton in other geographical areas (e.g. Nutrient-bearing water moves northward from Antarctic depths in summer, and there is algal blooming where that water wells up against the south-west African coast.) Seasonality adds dynamics to the oceans which receive energy from the sun.
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Since the end of the 19th century, our combustion engines have pumped 100-fold more carbon into the air than did civilizations prior to the Industrial Revolution. A small European car of the 20th century emits 1 kg of gaseous carbon in under 10 miles. (Currently, it is claimed that human activities are causing about 150-times more CO₂ emission than volcanoes could produce. Ref: skscience)
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Now that we have more biology, bio-geography and palaeontology, it can be deduced that 'aliens' are figments of our imagination.
Sagan's campaign to look for life in space resulted in the scrapping of a major part of the Voyager 1 mission. That space probe was diverted from her scheduled bypass of Jupiter, to take photos of one of Saturn's moons instead. Titan was said to be an 'Earth analog' (see "close look".) Voyager 2 continued alone during the rare planetary alignment, taking photos of Saturn, Uranus and Neptune, but her flight path put Neptune in the way of reaching Pluto. Voyager 1 would also have obtained images of those planets, and then would have provided some of Pluto if she hadn't been steered off-course. (Would NASA's "carbon-based lifeforms" have been eager to waste so much of their planetary alignment opportunity if they hadn't been exposed to so much hype from a Network television personality? )
Given the right elements, molecules will form and then assemble into living cells?
There seems to be no life without the cell, with the exception of a virus which can only exist if there are cells around. Molecules in a cell are built from carbon, hydrogen, oxygen, nitrogen (C, H, O, N) and other elements that confer useful functions. The molecules of life range from 'simple' carbon chains (e.g. sugars) to complex proteins (e.g. enzymes) in which nitrogen has a defining presence. (Although nucleotides might seem 'simpler' than protein and fat molecules are, they form a massive number of linear combinations in their DNA configuration.)
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| A nucleotide. |
Can life appear spontaneously if certain physical conditions are met? The molecular units that proteins are built of, known as amino acids, have occupied the attention of people who would demonstrate that they can take form spontaneously under 'the right conditions'. It's been said that molecules with some similarities to amino acids appeared when asteroid impact was simulated in a lab, and the lab assistant exclaimed on TV that "the building blocks of life" had been formed.
Even if amino acids can be found on a planet after an asteroid impact, they alone are not an indicator of life. Without the presence of nucleotides there is no coding. Nucleotide sequence provides coding for the production of all other molecules of a cell. Two copies of the coding in a DNA nucleotide sequence are created when the double-helix molecule undergoes replication, making reproduction of the organism possible.
Life's evolution was affected by geologic events and by asteroid impact.
The first living organisms are suggested to have been chemotrophic prokaryotes that can harness energy from chemical reactions where the products of volcanic activity are released into water. Such organisms changed our atmosphere in ways that made life possible for others which evolved later: See first-breath.
Fossil records show that Earth's inventory of life-forms through time has been shaped, sometimes traumatically, by environmental events.
The geological history of any planet is unique. Even if 'similar planets' do exist, they will differ at least as much as one person's fingerprint differs from any other's. This means that, if life could have appeared on another watery planet at the same time that it did on Earth (3.8 billion years ago), it would have followed different evolutionary pathways since then. For example, we know that, only 66 million years ago, the aftermath of an asteroid strike killed all dinosaurs except those that can rest for long periods under water. (Crocodilians might have survived because they are able to submerge for six hours. Some terrapins and snakes can also do voluntary apnoea.) With most of Earth's terrestrial dinosaurs being made extinct by the asteroid, small, warm-blooded mammals suddenly had an opportunity to proliferate: The Age of Mammals had been made possible. This collision of K-Pg and Earth was a unique event: there was no likelihood that an asteroid of similar size was hitting an 'identical' planet at the same time somewhere else in the universe.
It's remarkable that just one asteroid had such a fateful impact during the evolution of animals (and plants.) 'Not long' before life first appeared, there was a period in which millions of asteroids collided with Earth. See: Late_heavy_bombardment
Surely, every 'Earth-analog' has a unique history of asteroid strikes. Therefore, no planet but Earth has had the evolution that we have come to perceive: e.g. One that could produce a variety of mammals because the dinosaurs were removed at a key moment.
A big asteroid strike occurred in Scotland 1.2 billion years ago (see scotast.) This is when multicellular organisms first appear in the fossil record. It's tempting to wonder if there was a connection between the two events.
Even if there are two planets in the universe that are almost identical, just differences in the timing of asteroid strikes will ensure that only one of them would be home to the evolutionary pathways that produce sentient primates. There must be many fateful events in the evolution of any species of living organism, not just asteroid strikes e.g. continental drift, floods, drought.
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| Identical clouds are not seen. Identical super novae are not seen. Duplicate planets are just as unlikely. 'Gravitational lensing' explains the only known appearance of twin objects in space. |
In the universe, there cannot be identical sequences of asteroid strikes on any two planets (even if there are similar solar systems and a planet or two like Earth.) Therefore, there is only one planet with an evolution that produced the array of life forms we see on Earth today (broadly speaking: protozoa, coelenterates, echinoderms, flatworms, segmented worms, nematodes, arthropods, molluscs, fishes, sharks, amphibians, reptiles, birds, mammals; single-celled algae, multi-cellular algae, fungi, mosses, ferns, cycads, gymnosperms and angiosperms.) Given the singularity of K-Pg, the dinosaur-terminating asteroid of the Cretaceous-Paleogene, there is definitely only one planet which has large mammals. It's even more unlikely that there could be another planet which has mammalian primates, with opposable thumb and fore-finger, developed fore-brain (doing symbolic reasoning), society and culture: all essential for the emergence of technology, including space-craft.
