Multicellular, macroscopic algae and animals came onto the scene about million years ago. It's very hard to say how likely we are to find other intelligent life forms. The oldest fossils of our species, Homo sapiens, are less than , years old. That's a geological microsecond, just 0. If, like on Earth, intelligent life takes much longer to evolve than microbial life, it might be rare.
But some galaxies and solar systems are much older than ours, so maybe other intelligent life has formed; however, it may be long dead. Finding living intelligent beings is a matter of both space and time. Left Evidence that there may have once been liquid water on the surface of mars. Right Fossilized remains of underwater colonies of bacteria, called stromatolytes.
Just as conditions on Earth have changed drastically over time, so have conditions on other planets. When astrobiologists evaluate a planet, they look not just at its current conditions, but for signs that it could have been habitable in the past.
Liquid water leaves behind a kind of footprint. Mars, for example, has what looks like dry river and lake beds, making astrobiologists wonder if the cold, dry planet could have once been covered with life.
If it was, there may be enough remaining liquid water to sustain microbial life deep underground or under the frozen ice caps. Living things change their environment, leaving behind certain physical and chemical signatures.
The earth is covered by three-fourth of the water. What is Tide, why it occurs and how it is important for human life? The Sun's gravitational pull keeps our planet orbiting the Sun and the Earth firmly hold everything from topography to all forms of life through its gravitational pull. Hence, we can say, Earth is the only planet which hosts liquid water on its surface, allowing for a direct transfer of energy from the sun in order for photosynthetic life forms to evolve.
It has unique surface which neither too hot nor too cold for the occurrence of complex biochemical process. Basic Concepts in Geography. Are you worried or stressed? Click here for Expert Advice. Comment 0. Post Comment.
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There are a few key ingredients that scientists often agree are needed for life to exist — but much debate remains as to what limits there actually might be on life. Even Earth hosts some strange creatures that live in extreme environments. Here's what makes life able to thrive on our home planet and likely for alien life to arise on other worlds :. In such a soup, the ingredients for life as we know it, such as DNA and proteins, can swim around and interact with each other to carry out the reactions needed for life to happen.
The most common contender brought up for this solvent is the one life uses on Earth: water. Water is an excellent solvent, capable of dissolving many substances.
It also floats when it is frozen, unlike many liquids, meaning that ice can insulate the underlying fluid from freezing further. If water instead sunk when frozen, this would allow another layer of water to freeze and sink, and eventually all the water would get frozen, making the chemical reactions behind life impossible. Astronomers looking for extraterrestrial life most often focus on planets in the so-called habitable zones of their stars — orbits that are neither too hot nor too cold for liquid water to persist on the surfaces of those worlds.
Earth happened to hit the Goldilocks mark, forming within the sun's habitable zone. Mars and Venus lie outside it; if Earth's orbit had been just a bit further inside or outside of where it is, life may likely never have arisen and the planet would be a cold desert like Mars or a cloudy furnace like Venus.
Astrobiologists increasingly suggest looking beyond conventional habitable zones. For instance, while liquid water might not currently persist on the surface of Mars or Venus, there may have been a time when it did.
Life might have evolved on their surfaces in that time, and then either fled to safer locales on those planets, such as underground, or adapted to the environment when it became harsh, much as so-called extremophile organisms have on Earth, or both.
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