100,000 light years in diameter and 3,000 light years in width.

d - distance from Earth in light years (ly).

According to research, the universe is approximately 13.8 billion years old.

From fossil evidence, it appears that life may have existed on Earth as early as 3.5 billion years ago. This suggests that life must have evolved sometime during Earth's tumultuous first billion years.

"We tend to think of protists (single-celled organisms) as 'simple' and humans as 'complex' -- but the Naegleria genome shows us that much of this complexity arose really early in evolution," said Scott Dawson, assistant professor of microbiology at UC Davis. Dawson is senior author on the paper analyzing the genome of Naegleria gruberi, published in the March issue of the journal Cell. The team also included UC Davis graduate students Michael Cipriano and Jonathan Pham. Dawson had initially proposed N. gruberi as a candidate for genome sequencing back in 2004 while he was a postdoctoral researcher at UC Berkeley. N. gruberi slurps around in mud as an amoeba but when food runs low it sprouts two whip-like tails, or flagellae, and swims rapidly away. It can also transform into a hard, resistant cyst to wait out bad conditions. Most previous efforts to sequence the genomes of protozoa have focused on parasitic organisms such as the malaria parasite. "Because it's free living, it can tell us a lot about early life -- it has genes to do all these different things," Dawson said. The analysis shows that N. gruberi has 15,727 genes that code for proteins, compared to about 23,000 in humans. With those genes, the organism can eat and reproduce, crawl or swim, live with or without oxygen, and organize itself internally much as a human cell does.

Short lengths of RNA, could reproduce themselves like DNA, and might eventually build up to DNA. One can not make nucleic acids in the laboratory, from non-living material, let alone RNA. But given 500 million years, and oceans covering most of the Earth, there might be a reasonable probability of RNA, being made by chance. As DNA reproduced itself, there would have been random errors. Many of these errors would have been harmful, and would have died out. Some would have been neutral. That is they would not have affected the function of the gene. Such errors would contribute to a gradual genetic drift, which seems to occur in all populations. And a few errors would have been favourable to the survival of the species. These would have been chosen by Darwinian natural selection. The process of biological evolution was very slow at first. It took two and a half billion years, to evolve from the earliest cells to multi-cell animals, and another billion years to evolve through fish and reptiles, to mammals. But then evolution seemed to have speeded up. It only took about a hundred million years, to develop from the early mammals to us. The reason is, fish contain most of the important human organs, and mammals, essentially all of them. All that was required to evolve from early mammals, like lemurs, to humans, was a bit of fine-tuning.

One can define Life to be an ordered system that can sustain itself against the tendency to disorder, and can reproduce itself.

Possibility 2) The galaxy has been colonized, but we just live in some desolate rural area of the galaxy. The Americas may have been colonized by Europeans long before anyone in a small Inuit tribe in far northern Canada realized it had happened. There could be an urbanization component to the interstellar dwellings of higher species, in which all the neighboring solar systems in a certain area are colonized and in communication, and it would be impractical and purposeless for anyone to deal with coming all the way out to the random part of the spiral where we live.

As many stars as there are in our galaxy (100 - 400 billion), there are roughly an equal number of galaxies in the observable universe -- so for every star in the colossal Milky Way, there's a whole galaxy out there. All together, that comes out to the typically quoted range of between 1022 and 1024 total stars, which means that for every grain of sand on Earth, there are 10,000 stars out there.

Of the 1,780 confirmed planets beyond our solar system, as many as 16 are located in their star’s habitable zone, where conditions are neither too hot nor too cold to support life. Size also matters: A planet that’s too small can’t maintain an atmosphere; one that’s too large will have a crushing atmosphere. A recently detected planet 493 light-years from Earth, Kepler-186f, is close to Earth's size and is located in its solar system's habitable zone.

Of the 1,780 confirmed planets beyond our solar system, as many as 16 are located in their star’s habitable zone, where conditions are neither too hot nor too cold to support life.

The Drake equation, formulated in 1961, estimates the number of alien civilizations we could detect. Recent discoveries of ­numerous planets in the Milky Way have raised the odds.

These two sites—a frozen Arctic lake and a to­xic tropical cave—could provide clues to one of the oldest, most compelling mysteries on Earth: Is there life beyond our planet? Life on other worlds, whether in our own solar system or orbiting distant stars, might well have to survive in ice-covered oceans, like those on Jupiter's moon Europa, or in sealed, gas-filled caves, which could be plentiful on Mars. If you can figure out how to isolate and identify life-forms that thrive in similarly extreme surroundings on Earth, you're a step ahead in searching for life elsewhere.

These two sites—a frozen Arctic lake and a to­xic tropical cave—could provide clues to one of the oldest, most compelling mysteries on Earth: Is there life beyond our planet? Life on other worlds, whether in our own solar system or orbiting distant stars, might well have to survive in ice-covered oceans, like those on Jupiter's moon Europa, or in sealed, gas-filled caves, which could be plentiful on Mars.