Life is often defined as a property which differentiates the dead and alive! Those who perform life processes, like transportation, digestion, respiration etc. are classified as livings and those that do not, either because such functions have ceased, or because they never had such functions and are classified as non-livings. However, several other definitions of life have been proposed, and there are some ‘borderline’ cases of life (which are neither completely living nor dead), such as viruses.
Possible Beginning of Life
Currently, we believe that life came from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but a gradual process of increasing complexity. Life on Earth first appeared as early as 4.28 billion years ago, soon after ocean formation 4.41 billion years ago, and not long after the formation of the Earth 4.54 billion years ago. The earliest known life forms are micro-fossils of bacteria.
Researchers generally think that current life on Earth descends from an RNA world, although RNA-based life may not have been the first life to have existed. The classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Complex organic molecules occur in the Solar System and in interstellar space, and these molecules may have provided starting material for the development of life on Earth.
The modern definition of abiogenesis, however, is concerned with the formation of the simplest forms of life from primordial chemicals, rather than the old Aristotelian concept of abiogenesis, which postulated the formation of fully-formed complex organisms by spontaneous generation.
It becomes, then, the search for some kind of molecule (along the lines of RNA or DNA) that is simple enough that it can be made by physical processes on the young Earth, yet complicated enough that it can take charge of making more of itself, which is probably what most people would recognize as constituting “life”.
Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale, but it has also adapted to survive in most ecosystems and conditions. Some microorganisms, called extremophiles, thrive in physically or geo-chemically extreme environments that are detrimental to most other life on Earth.
It is presumed that, over a few hundred million years of evolution, pre-biotic molecules evolved into self-replicating molecules by natural selection. While some aspects of the subject are well understood, others remain clouded in mystery and are the source of much contention among scientists. Although much progress has been made, there is still no single definitive theory.
The cell is considered the structural and functional unit of life. There are two kinds of cells, prokaryotic and eukaryotic, both of which consist of cytoplasm enclosed within a membrane and contain many bio-molecules such as proteins and nucleic acids. Cells reproduce through a process of cell division, in which the parent cell divides into two or more daughter cells.
Life is Physics
Physicists are on the hunt for a “theory of life” that explains why life can exist.
Organisms have aspects of both complexity and order, as in this slice of a plant stem. Researchers hope to eventually develop basic equations that describe all of life.
There is nothing simple about life. Millions of carefully coordinated chemical reactions occur every second inside a single cell; billions of single-celled organisms can organize into colonies; trillions of cells can precisely stick together into tissues and organs. Yet, despite this complexity, life is easy to identify. Physicists think that this recognizability could arise from foundational physical principles that underlie all life. And they are on the hunt for mathematical formulations based on these principles that explains why life can exist and how it behaves. Such a theory, they say, could allow researchers to control and manipulate living systems in ways that are currently impossible.
Physicists love unifying theories. These theories boil complex phenomena down to a small set of ideas whose mathematical formulations can make remarkably successful predictions. For example, the laws of thermodynamics, which explain how energy moves around in systems from atoms to hurricanes, can accurately predict how long a kettle of water takes to boil. Yet despite such successes, researchers have not yet found universal equations that describe everyday phenomena relating to life. Such equations could provide the same predictive power as other unifying theories, allowing researchers to gain precise control over living things.
This control could enable better treatment protocols for bacterial infections, improved therapies for cancers, and methods to prevent plants from developing resistance to weed killers.
“Physicists have studied many complicated systems, but living systems are in a completely different class in terms of complexity and the number of degrees of freedom they have”says Ramin Golestanian, a director at the Max Planck Institute for Dynamics and Self-Organization in Germany.
Golestanian studies living systems, like bacterial swarms, by modeling them as moving groups of energy-consuming particles, so-called active matter. He also helped organize Physics of Living Matter, an APS conference held last year, where researchers discussed whether writing down a mathematical theory of life is an achievable goal and, if so, what questions such a theory should answer.
Life on Mars
Recently, NASA’s Mars Curiosity rover detected “tough” organic molecules in 3-billion-year-old sedimentary rocks within five centimeters of the surface, at least one prominent planetary scientist thinks that the debate over whether Mars first seeded Earth with life or vice-versa will only intensify.
The findings appeared in last week’s issue of the journal Science along with a second paper which noted that Curiosity has also detected seasonal variations in minuscule amounts of Mars’ atmospheric methane.
But the $64,000 question remains: if life arose on Mars did it do so independently? Or did one planet seed the other through the meteoritic exchange of organics or even biota? This is what the ultimate conundrum, Cornell University planetary scientist Jonathan Lunin said.
For as some astrobiologists have long argued, if we find evidence that life arose independently on Mars – only the next planet out, then it’s only logical to conclude that life in the cosmos is very common indeed.
“Curiosity struck organic gold in Gale Crater because it was once a lake environment, where organics would have been concentrated and preserved in sediments,”Lunine Said
NASA reports that some of the molecules identified include thiophenes, benzene, toluene, and small carbon chains, such as propane or butene.
The sulphur that is dominant in these organics stabilizes them, greatly enhancing the possibility that they would survive in the soil for billions of years,”Lunine Said
And given the evidence for habitable environments that may have lasted for hundreds of millions of years, life may have begun on Mars, Lunine says. But the exchange of microbes with Earth through large impacts, early in Mars’ history, might have cross-contaminated the two planets, he says.
But did life on our two planets actually first originate on Mars?
“This is the dilemma,” said Lunine. “Mars and Earth are close enough to have exchanged lots of material over the age of the solar system.”
But as noted here previously, some researchers think that both ultraviolet radiation from the young Sun and galactic cosmic rays would have likely destroyed microbial life in the unprotected vacuum of space. And even if microbial life survived the journey to Earth, it’s doubtful it would have survived the trip through Earth’s atmosphere and then adapted to its new home.
Even so, Lunine counters that it’s too soon to say whether or not biota were shared. And even if we find life, these arguments will persist unless we find a living cell. Although he notes that is very unlikely, he says it would be required for researchers to be able to study the biochemistry of putative Martian life.
“This is why I am keenly interested in Saturn’s moon of Enceladus; it’s far enough away that interplanetary transfer of any such ancient life into the inner solar system would have been much less likely,”Lunine Said
Although NASA says that while Curiosity has not determined the source of the organic molecules, data collected by the rover reveals that Gale Crater once held all the ingredients needed for life.
What are we missing in our current search for ancient and/or extant life on Mars?
Measuring the isotopic ratio of carbon in the gaseous methane—a measurement that requirse great sensitivity—would help to constrain whether that methane is produced by water reacting with carbon dioxide and rock or by biology, says Lunine.
NASA’s Mars 2020 rover which should land on Mars in 2021, says Lunine, has an instrument payload that can detect organic compounds and look for chemical and imaging indications of life on millimeter scales. And the European Space Agency’s (ESA) ExoMars program includes ongoing orbital measurements to help map Mars’ methane, he says. The ExoMars rover will also look for life in samples that will be recovered from six-foot drills.
“This will be an excellent follow-on to Curiosity,” said Lunine.
Future and End of Life
The biological and geological future of Earth can be extrapolated based upon the estimated effects of several long-term influences. These include the chemistry at Earth’s surface, the rate of cooling of the planet’s interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun’s luminosity.
An uncertain factor in this extrapolation is the ongoing influence of technology introduced by humans, such as climate engineering, which could cause significant changes to the planet. The current Holocene extinction is being caused by technology and the effects may last for up to five million years. In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.
Over time intervals of hundreds of millions of years, random celestial events pose a global risk to the biosphere, which can result in mass extinctions. These include impacts by comets or asteroids, and the possibility of a massive stellar explosion, called a supernova, within a 100-light-year radius of the Sun.
Other large-scale geological events are more predictable. Milankovitch theory predicts that the planet will continue to undergo glacial periods at least until the “Quaternary glaciation” comes to an end. These periods are caused by variations in eccentricity, axial tilt, and precession of the Earth’s orbit. As part of the ongoing supercontinent cycle, plate tectonics will probably result in a supercontinent in 250–350 million years. Sometime in the next 1.5–4.5 billion years, the axial tilt of the Earth may begin to undergo chaotic variations, with changes in the axial tilt of up to 90°.
The luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching the Earth. This will result in a higher rate of weathering of silicate minerals, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years from now, the level of carbon dioxide will fall below the level needed to sustain C3 carbon fixation photosynthesis used by trees. Some plants use the C4 carbon fixation method, allowing them to persist at carbon dioxide concentrations as low as 10 parts per million. However, the long-term trend is for plant life to die off altogether. The extinction of plants will be the demise of almost all animal life, since plants are the base of the food chain on Earth.
In about one billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a “moist greenhouse”, resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics will come to an end, and with them the entire carbon cycle. Following this event, in about 2–3 billion years, the planet’s magnetic dynamo may cease, causing the magnetosphere to decay and leading to an accelerated loss of volatiles from the outer atmosphere. Four billion years from now, the increase in the Earth’s surface temperature will cause a runaway greenhouse effect, heating the surface enough to melt it. By that point, all life on the Earth will be extinct. The most probable fate of the planet is absorption by the Sun in about 7.5 billion years, after the star has entered the red giant phase and expanded beyond the planet’s current orbit.
As the Sun orbits the Milky Way, wandering stars may approach close enough to have a disruptive influence on the Solar System. A close stellar encounter may cause a significant reduction in the perihelion distances of comets in the Oort cloud—a spherical region of icy bodies orbiting within half a light year of the Sun. Such an encounter can trigger a 40-fold increase in the number of comets reaching the inner Solar System. Impacts from these comets can trigger a mass extinction of life on Earth. These disruptive encounters occur at an average of once every 45 million years. The mean time for the Sun to collide with another star in the solar neighborhood is approximately 3 × 1013 years, which is much longer than the estimated age of the Milky Way galaxy, at ~1.3 × 1010 years. This can be taken as an indication of the low likelihood of such an event occurring during the lifetime of the Earth.
The energy release from the impact of an asteroid or comet with a diameter of 5–10 km (3–6 mi) or larger is sufficient to create a global environmental disaster and cause a statistically significant increase in the number of species extinctions. Among the deleterious effects resulting from a major impact event is a cloud of fine dust ejecta blanketing the planet, blocking some direct sunlight from reaching the Earth’s surface thus lowering land temperatures by about 15 °C (27 °F) within a week and halting photosynthesis for several months. The mean time between major impacts is estimated to be at least 100 million years. During the last 540 million years, simulations demonstrated that such an impact rate is sufficient to cause 5–6 mass extinctions and 20–30 lower severity events. This matches the geologic record of significant extinctions during the “Phanerozoic Eon”. Such events can be expected to continue into the future.
The incremental effect of gravitational perturbations between the planets causes the inner Solar System as a whole to behave chaotically over long time periods. This does not significantly affect the stability of the Solar System over intervals of a few million years or less, but over billions of years the orbits of the planets become unpredictable. Computer simulations of the Solar System’s evolution over the next five billion years suggest that there is a small (less than 1%) chance that a collision could occur between Earth and either Mercury, Venus, or Mars. During the same interval, the odds that the Earth will be scattered out of the Solar System by a passing star are on the order of one part in 105. In such a scenario, the oceans would freeze solid within several million years, leaving only a few pockets of liquid water about 14 km (8.7 mi) underground. There is a remote chance that the Earth will instead be captured by a passing binary star system, allowing the planet’s biosphere to remain intact. The odds of this happening are about one chance in three million.