If you've ever shopped for real estate, you've heard the phrase location, location, location in reference to what's most important about a home. The same can be said of planet earth. The fact that we are doing physics together means humanity came about. While you might think that fact to be nothing special, I would like you to think again.[br][br]According to what we know of fundamental physics that governs the universe, the universe formed in a sudden flash of light in an event we call the [b]big bang[/b]. All the evidence we have says that it was a one-time event which will never repeat itself. [br][br]Within that universe scientists, regardless of their philosophical views, agree that there is some uncanny fine tuning of the constants of nature. We can with physics literally calculate the structure of atoms and nuclei and planets and stars, etc. If things like the gravitational constant were a little different, or the fundamental charge of the electron and proton were different, or any of a long list of constants were different, we would not have a universe like we know it. Stars wouldn't have formed, atoms wouldn't exist (much less molecules). [br][br]I do not wish to get into the philosophical discussions about this fine tuning except to mention it. I will, however, say the following: People have strong opinions about this fine tuning because such tuning either implies winning a cosmic lottery against extraordinary odds, or a deliberate action by some creator. As of this writing, there are no scientific explanations for this tuning, and all that seem to be scientific are really philosophical speculation and not testable scientific hypotheses. [br][br]The fact according to testable science is that the universe is a very unlikely one-time event. But for there to be a universe is quite a different thing than for that universe to contain life. When we read of evolutionary biology, science tells us of another one-time event. [br][br]While it is often said that life was bound to occur on a planet such as ours with all the appropriate ingredients for life, what's undisputed by science is the fact that the tree of life - technically called the phylogenetic tree - started with one common ancestor referred to as the LUCA (last universal common ancestor). The event leading to what we call life was a one-time event as well, according to empirical science. This should be thought provoking. Was life really bound to occur if there is only a single organism at the root of the phylogenetic tree? If life was bound to occur, there would be many ancestors of life perhaps all with unique chemistry. Looking at the data and avoiding speculation says this: Life occurred at one time in history at one place. It was a singular event. Any speculation otherwise is only speculation for philosophical reasons. There is no scientific evidence suggesting it. [br][br][b]As far as science knows, the universe is a one-time event, and so is life.[/b] We have been searching for other signals that might indicate other intelligent life in space since the 1960s at SETI (search for extra-terrestrial intelligence) and have come up empty handed. It would be exciting to find other complex lifeforms, but so far we have not.[br][br]Life is extraordinarily delicate. By their nature, molecules of life need to be stable enough to not just fall apart, but unstable enough to fulfill their biological function which requires structural rearrangement not unlike moving parts in machines. Getting a high fever is dangerous to our health because only a few degrees centigrade increase in body temperature will literally start to melt the delicate molecules within us.[br][br]With this in mind it stands to reason that life needed just the right place to form - especially complex intelligent life - because with complexity comes increased delicacy unfortunately. There are reasons that are often overlooked that suggest that life will be harder to find outside of earth than we think. We read of "earth-like" planets being discovered in the news several times per year at least. Earth-like is used loosely in those articles. Here are some requirements for life according to science:[br][list=1][*]Liquid water: Water regulates earth's climate by having an unusually large heat capacity. It also serves biological function. This requirement determines the proximity and type of star that we orbit, as well as the eccentricity of the orbit. Liquid water is the only substance known to science that can play a unique role as a moderator of temperature, a weak solvent and a transport mechanism all at the same time... and our planet is covered by it. Life is dependent on it. Thus, the search for life elsewhere begins with the search for locations allowing for liquid water.[/*][*]Correct planetary density and composition: The planet's gravity determines the make-up of the atmosphere. If gravity is too weak, the atmosphere will be only heavy molecules if it has one at all. Too strong and many light and/or toxic molecules make up the atmosphere. [/*][*]A large moon: It can be calculated that a single, large moon is required to stabilize a planet's axis of rotation against precession from surrounding gravitational effects. Precession would upset the global climate catastrophically.[/*][*]Location within galaxy: Our location needs to be far from the galactic core and outside of a dense spiral arm if long term survival of life is expected.[/*][*]Large outer planets: Jupiter, had it been a bit larger, would have ignited fusion and become another sun. That would have prevented life on earth. The majority of all the stars in the sky around which there could be other solar systems, in fact are binary systems with two stars. Instead, in our case, Jupiter's enormous mass and gravitation does the job of solar system junk collection of incoming comets and other debris that could end life on earth. It is suggested from modeling that such an arrangement like ours is necessary for long-term survival. [/*][*]Active geology and tectonic plates seem to be necessary to foster renewal or recycling of resources.[/*][*]Carbon is necessary for its diverse chemistry - which is physics :) No other element allows for comparable diversity as is required to construct the molecules of life. [/*][/list][br]After reading this, I hope you feel extraordinarily fortunate! Nobody like you was ever born in this one-time event called life, in this one-time "experiment" called the universe.