Category Science

Nobody knows WHAT conditions are needed for life to begin. Some scientists have suggested that living cells may have been brought to Earth by a comet. When the Giotto probe investigated Halley’s Comet in 1986, it found molecules that were similar to living cells. If a comet like this collided with Earth at the right time, then life may have taken hold. Another theory is that powerful lightning bolts flashing through Earth’s early atmosphere may have caused chemical reactions, which created living cells.

One of the first ideas, popularised by biochemist Sidney Fox in the wake of the Miller-Urey experiment, was that amino acids assembled into simple proteins. In modern organisms, proteins perform a huge range of functions, including acting as enzymes that speed up essential chemical reactions. However, this proteins-first hypothesis has largely fallen out of favour.

A much more popular notion is that life began with RNA, a close cousin of DNA, in an “RNA World”. RNA can carry genes and copy itself just like DNA, but it can also fold up and act as an enzyme, just like a protein. The idea was that organisms based solely on RNA arose first, and only later developed DNA and protein.

The RNA World has amassed a lot of supporting evidence, but it is not clear that RNA alone was enough. In recent years, some researchers have suggested that RNA only really reaches its potential when it is paired with proteins – and that both must have existed for life to get started.

A third school of thought is that the first organisms were simple blobs or bubbles. These “protocells” would have resembled modern cells in one key attribute: they acted as containers for all the other components of life. More advanced protocells developed by the Nobel Prize winning biologist Jack Szostak also contain self-replicating RNA.

The final hypothesis is that life began with a series of chemical reactions that extracted energy from the environment and used that energy to build the molecules of life. This “metabolism-first” idea was championed in the late 1980s by Günter Wachtershauser, a German chemist turned patent lawyer. Wachtershauser envisioned a series of chemical reactions taking place on crystals of iron pyrite (“fool’s gold”), a scheme he dubbed the “Iron-Sulphur World”. However, nowadays this idea has been supplanted by Michael Russell’s suggestion that the first life was powered by currents of electrically-charged protons within alkaline vents on the sea bed.

While we cannot know for sure which of these scenarios played out on our planet, successfully creating life from chemicals in the lab would at least tell us which of the proposed mechanisms actually works. 

The ecosphere is a narrow band around the Sun where the temperature is neither too hot nor too cold for life to exist. Earth is the only planet in this zone, and is therefore the only planet in the Solar System able to support life. Mercury and Venus are too close to the Sun for water to exist in liquid form. The remaining planets lie well beyond the ecosphere, where it is too cold for life. The temperature on Pluto can reach as low as —223 °C (-370°F)!

An ecosphere is a planetary closed ecological system. In this global ecosystem, the various forms of energy and matter that constitute a given planet interact on a continual basis. The forces of the four Fundamental interactions cause the various forms of matter to settle into identifiable layers. These layers are referred to as component spheres with the type and extent of each component sphere varying significantly from one particular ecosphere to another. Component spheres that represent a significant portion of an ecosphere are referred to as a primary component spheres. For instance, Earth’s ecosphere consists of five primary component spheres which are the Geosphere, Hydrosphere, Biosphere, Atmosphere, and Magnetosphere.

Like all stars, our Sun will eventually die. In around five billion years its supply of hydrogen will run out, and it will become a red giant, expanding to well over thirty times its current size. As it grows, the Sun will engulf all the inner planets, making them far too hot for life to survive.

There’s nothing we can do to prevent this cataclysm. Yet according to scientists who study the far future, including Yale University astronomer Gregory Laughlin, the prospect for life is, oddly, rather bright. Given technological advances and the continuing evolution of our species, humans should be able to survive — in some form — long after Earth has ceased to exist.

But our distant descendants are going to have to do some planet-hopping. The first major cosmic crisis will strike in about 1.5 billion years. At that point, according to projections by environmental scientist Andrew J. Rushby at the University of East Anglia in England, the brightening sun will set off what might be termed “super-global” warming. Earth will be heated until the oceans boil.

By then, though, will we care? We already have the technology to establish bases on the moon and Mars. So a billion and a half years from now, we’ll likely have colonized the whole solar system — and perhaps other star systems in our Milky Way galaxy.

As the sun grows hotter, other planets will become more appealing. Just as Earth becomes too toasty to sustain life, Mars will reach a temperature that makes it habitable. Cornell University astronomer Lisa Kaltenegger has run models showing that the Red Planet could then stay pleasant for another 5 billion years.

About 7.5 billion years from now, the sun will exhaust its hydrogen fuel and switch to helium. That will cause it to balloon into an enormous red giant. Mars as well as Earth will be fried. On the other hand, the once icy moons of Jupiter and Saturn will have become tropical water worlds — prime real estate for human colonies. We could live there for a few hundred million years.

About 8 billion years from now, the flaring sun will make conditions intolerably hot all the way out past Pluto. “The exact dates depend on how much mass you estimate the sun will lose and how much planets will move,” Kaltenegger says. But the message is clear: Life will be impossible in our solar system.

For much of its early history, Earth was a bubbling, volcanic ball — far too hot to sustain life. Over millions of years, the surface of the planet began to cool and harden, releasing enormous clouds of steam and gas. The moisture in these clouds eventually became rain, forming the seas. Scientists believe that the first life-forms originated in shallow pools of water, where different chemicals were concentrated to form single-celled organisms. These gradually evolved into more complex life-forms. All living creatures on Earth are still evolving.

Microbial life forms have been discovered on Earth that can survive and even thrive at extremes of high and low temperature and pressure, and in conditions of acidity, salinity, alkalinity, and concentrations of heavy metals that would have been regarded as lethal just a few years ago. These discoveries include the wide diversity of life near sea–floor hydrother­mal vent systems, where some organisms live essentially on chemical energy in the absence of sunlight. Similar environments may be present elsewhere in the solar system.

Under­standing the processes that lead to life, however, is complicated by the actions of biology itself. Earth’s atmosphere today bears little resemblance to the atmosphere of the early Earth, in which life developed; it has been nearly reconstituted by the bacteria, vegetation, and other life forms that have acted upon it over the eons. Fortunately, the solar system has preserved for us an array of natural laboratories in which we can study life’s raw ingredients — volatiles and organics — as well as their delivery mechanisms and the prebiotic chemical processes that lead to life. We can also find on Earth direct evidence of the interactions of life with its environments, and the dramatic changes that life has undergone as the planet evolved. This can tell us much about the adaptability of life and the prospects that it might survive upheavals on other planets.

Earth is the only place in the Solar System on which scientists have encountered life. Conditions on our planet are perfect for sustaining life — the surface temperature averages around 15°C (59°F) , allowing water to exist in liquid form. Water is a vital ingredient for life, and its presence on Earth has enabled an incredible variety of creatures to live on every part of the planet. Also, Earth is large enough to contain a protective atmosphere, but not big enough to become a suffocating gas planet like Jupiter or Saturn.

Although the exact process by which life formed on Earth is not well understood, the origin of life requires the presence of carbon-based molecules, liquid water and an energy source. Because some Near-Earth Objects contain carbon-based molecules and water ice, collisions of these objects with Earth have significant agents of biologic as well as geologic change.

For the first billion years of Earth’s existence, the formation of life was prevented by a fusillade of comet and asteroid impacts that rendered the Earth’s surface too hot to allow the existence of sufficient quantities of water and carbon-based molecules. Life on Earth began at the end of this period called the late heavy bombardment, some 3.8 billion years ago. The earliest known fossils on Earth date from 3.5 billion years ago and there is evidence that biological activity took place even earlier – just at the end of the period of late heavy bombardment. So the window when life began was very short. As soon as life could have formed on our planet, it did. But if life formed so quickly on Earth and there was little in the way of water and carbon-based molecules on the Earth’s surface, then how were these building blocks of life delivered to the Earth’s surface so quickly? The answer may involve the collision of comets and asteroids with the Earth, since these objects contain abundant supplies of both water and carbon-based molecules.

Once the early rain of comets and asteroids upon the Earth subsided somewhat, subsequent impacts may well have delivered the water and carbon-based molecules to the Earth’s surface – thus providing the building blocks of life itself. It seems possible that the origin of life on the Earth’s surface could have been first prevented by an enormous flux of impacting comets and asteroids, then a much less intense rain of comets may have deposited the very materials that allowed life to form some 3.5 – 3.8 billion years ago.

Comets have this peculiar duality whereby they first brought the building blocks of life to Earth some 3.8 billion years ago and subsequent commentary collisions may have wiped out many of the developing life forms, allowing only the most adaptable species to evolve further. It now seems likely that a comet or asteroid struck near the Yucatan peninsula in Mexico some 65 million years ago and caused a massive extinction of more than 75% of the Earth’s living organisms, including the dinosaurs. At the time, the mammals were small burrowing creatures that seemed to survive the catastrophic impact without too much difficulty. Because many of their larger competitors were destroyed, these mammals flourished. Since we humans evolved from these primitive mammals, we may owe our current preeminence atop Earth’s food chain to collisions of comets and asteroids with the Earth.