Why are the conditions for alien life to evolve the same as ours? Why can’t they evolve without water, or extremely far from their sun? Is there a reason for this or is it just because our only example is ourselves?

Question

Idk if to put biology or planetary science so ye.

In: Biology

Answer 1

Hi!

Great question.

There are a few things to consider.

#Energy issues

The first is very unforgiving – we call it **thermodynamics or energy issues**.

When there is **too much energy** (say inside a sun) there is little chance of any physical substance staying organized in that high energy, chaotic environment.

When there is **too little energy** (say far from a sun and far from geologically produced heat) there is nothing to drive the chemical reactions that we think are needed for living.

#Chemistry issues

All the life we know of is based largely on the interesting chemistry of Hydrogen, Oxygen and Carbon. These are mostly in the form of water and organic (carbon based) molecules.

We can imagine that chemistry not based on these (inorganic chemistry) might be possible as a living system, but we have not yet seen it. We do have some life forms that get their energy by reducing iron or sulfur or other inorganic chemistry but the creature itself is carbon based.

In 2018, Michael Wall wrote a book [Out There: A Scientific Guide to Alien Life, Antimatter, and Human Space Travel](https://annas-archive.org/md5/601a84b2a78a885fb736c6ee8116bdf6) that is worth reading.

By mass our universe is 75% hydrogen, 23% Helium, 1% Oxygen, 0.5% carbon and all the rest of the periodic table making up the 0.5% left over. (Remember that Hydrogen and helium have the lightest mass, so them making up 98% of the universe by mass is a big deal.)

This suggests that Hydrogen and Oxygen and carbon should be major players in any form of life based on their abundance. Helium is the least chemically reactive element. Helium is unlikely to react in any conditions where we expect life to thrive.

#Earth – our only example

The physics of our location seems rare.

* We have a magnetosphere that blocks cosmic radiation so we are protected from that. DNA and other bits important to our kind of life require this.

* We have abundant liquid water. That is a big deal. Life on earth is all about chemistry in a water solution. We have cell membranes that keep water in, and keep water out. We are absolutely dependent of water for life.

* We have a narrow temperature range where we can survive. Using the Kelvin scale, (0C = 273K) it is hard for human life to thrive below 273k or above 320k.

* Most life also does not do well in hard vacuum, or at high pressures. The only life we know of seems adapted for or created for earth like conditions.