“Six numbers in particular govern our universe, and if any of these values changed even very slightly, things couldn’t be as they are. For example, for the universe to exist as it does requires that hydrogen be converted to helium in a precise but comparatively stately manner ― specifically in a way that converts seven one-thousandths of its mass to energy. Lower that value very slightly ― from 0.007 to 0.006 percent, say ― and no transformation could take place: the universe would consist of hydrogen and nothing else. Raise the value very slightly ― to 0.008 percent ― and bonding would be so wildly prolific that the hydrogen would long since have been exhausted. In either case, with the slightest tweaking of the numbers, the universe as we know and need it wouldn’t be here.” 
“What if the speed of light, rather than being 186,282 miles per second, was 185,000, or 187,000? Would that change much of anything? What if the force of gravity were one percent more or less than it is? The fundamental constants of physics ― the speed of light, the constant of gravitational attraction, the weak and strong forces of subatomic interaction, Planck’s constant ― have values that, of course, permit the actual development of the universe as we know it to have happened. But it turns out that if, in imagination, we change any of these values by just the tiniest amount, we thereby posit a universe in which none of this could’ve happened, and indeed in which apparently nothing life-like ever could have emerged: no planets, no atmospheres, no solids at all, no elements except hydrogen and helium, or maybe not even that ― just some boring, hot, undifferentiated stuff. So isn’t it a wonderful fact that the laws are just right for us to exist? Indeed, one might want to add, we almost didn’t make it!” [Daniel Dennett]
[from Wikipedia’s Anthropic Principle article]
“The anthropic principle is the philosophical consideration that observations of the universe must be compatible with the conscious and sapient life that observes it. Proponents of the anthropic principle argue that it explains why this universe has the age and the fundamental physical constants necessary to accommodate conscious life, since if either had been different, we wouldn’t have been around to make observations. Anthropic reasoning is often used to deal with the notion that the universe seems to be fine-tuned.
The principle was formulated as a response to a series of observations that the laws of nature and parameters of the universe take on values that are consistent with conditions for life as we know it rather than a set of values that wouldn’t be. The anthropic principle states that this is a necessity, because if life were impossible, no living entity would be there to observe it, and thus wouldn’t be known. That is, it must be possible to observe some universe, and hence, the laws and constants of any such universe must accommodate that possibility.”
Various Statements of the Anthropic Principle
- “We must be prepared to take account of the fact that our location (in space and time) in the universe is necessarily privileged to the extent of being compatible with our existence as observers.” 
- “Conditions that are observed in the universe must allow the observer to exist.”i
- “The universe, and hence the fundamental parameters on which it depends, must be such as to admit the creation of observers within it at some stage.” 
- “The universe’s fine-tuning is the result of selection bias, specifically survivor bias ― only in a universe capable of eventually supporting life will there be living beings capable of observing and reflecting upon fine-tuning.” 
- “Observers are necessary to bring the universe into being.” 
- “The universe is very old and very large. Humankind, by comparison, is only a tiny disturbance in one small corner of it ― and a very recent one. Yet the universe is only very large and very old because we’re here to say it is. And yet, of course, we all know perfectly well that it is what it is whether we’re here or not.” 
- “The universe (or a universe) is, in some sense, compelled to have conscious and sapient life emerge within it.” 
- “The conditional probability of finding yourself in a universe compatible with your existence is always 1.” 
- You are a self-reflective being, and all other facts about the (physical) universe are arbitrary/contingent.
“Some versions of the anthropic principle are merely tautologies, whereas others are not and thus make claims that are (considered) controversial.”
“Most versions of the anthropic principle draw upon some notion of a multiverse for there to be a statistical population of universes to select from, and from which selection bias (our observance of only this universe, compatible with our life) could occur. Postulating a multiverse is a radical step, but it could provide at least a partial answer to a question which has seemed to be out of the reach of science: ‘Why do the fundamental laws of physics take the particular form we observe and not another?’
If it’s granted that there’s not one universe but a whole infinite ensemble of universes with all possible initial conditions, then the anthropic principle provides a plausible explanation for the fine-tuning of our universe: the ‘typical’ universe isn’t fine-tuned, but given enough universes, a small fraction of them will be capable of supporting intelligent life. Ours must be one of these, and so the observed fine-tuning should be no cause for wonder.”
[from the Privileged Character of 3+1 Spacetime subsection of Wikipedia’s Spacetime article]
Many thinkers have proposed interesting but highly speculative theories that attempt to explain why our universe has 3 dimensions of space and 1 dimension of time by ruling out other options.
On a slight tangent, I remember once reading about the distinction between discrete and continuous, and for examples of discrete things found in nature, they listed atoms, cells, and dimensions of spacetime.
“There are two kinds of dimensions: spatial (bidirectional) and temporal (unidirectional). Let the number of spatial dimensions be S and the number of temporal dimensions be T. That S.=.3 and T.=.1, setting aside the compactified dimensions invoked by string theory and undetectable to date, can be explained by appealing to the physical consequences of letting S differ from 3 and T differ from 1.”
“In 1920, Paul Ehrenfest showed that if there’s only 1 time dimension and greater than 3 spatial dimensions, the orbit of a planet about its sun can’t remain stable. The same is true of a star’s orbit around the center of its galaxy. He also showed that if there are an even number of spatial dimensions, then the different parts of a wave impulse will travel at different speeds. In 1922, Hermann Weyl showed that Maxwell’s theory of electromagnetism works only with 3 dimensions of space and 1 of time. In 1963, Frank Tangherlini showed that when there are more than 3 spatial dimensions, electron orbitals around nuclei can’t be stable ― electrons would either fall into the nucleus or disperse.”
“Max Tegmark maintains that if T.>.1, protons and electrons would be unstable and could decay into particles having greater mass than themselves. And if S.<.3, the universe is probably too simple to contain observers ― for example, nerves can’t cross without intersecting.”
This is similar to the idea that, “a Flatlander (a hypothetical two-dimensional being) couldn’t possibly have a digestive tract, because the pipe from their mouth to their bottom would divide them in two pieces.” 
[Pannenkoek] “I bet if God tried really hard, He could make S.=.2 work.”
Once, I was talking to my philosophy professor about the possibility of life in a universe with only two dimensions of space (and one dimension of time). I mentioned that while maybe it’s possible with two, it’s definitely not possible with only one. In response, he said, “Well, imagine an infinite line of bits or lightbulbs that could be in two possible states: 0 or 1, off or on. Within this, there could be arbitrarily complex interactions and patterns.”i
I once dreamt that I was in a black void, possibly as a disembodied observer. In front of me, in the distance, were two horizontal ribbons of light ― one red, one blue ― stretching seemingly infinitely in both directions, with these incredibly intricate waves moving through them. Suddenly, I had this realization that the two ribbons of light were interacting or communicating or dancing ― though I couldn’t understand it. And the higher-order patterns formed by the waves were as complex as life, and consciousness itself. It was one of the most beautiful dreams I ever had.
“In general, it’s not clear how physical law could function if T differed from 1. If T.>.1, subatomic particles which decay after a fixed period would not behave predictably. S.=.1 and T.=.3 (the inverse of our universe) has the peculiar property that the speed of light in a vacuum is a lower bound on the velocity of matter; all matter consists of tachyons.”
- Bill Bryson
- Brandon Carter
- Merriam-Webster Online Dictionary
- John Barrow & Frank J. Tipler
- Michael Frayn
- Jürgen Schmidhuber
- Edwin Abbott
- Andy Norman