"... why would humans fight, kill, invest, deprive themselves and expend vast amounts of material and energy into building giant cathedrals and embarking on lengthy missions throughout our history???"
Your question covers too much ground but here goes:
1. Fighting, killing, and mega structures.
The reason for both is rooted in our biological propensity to dominate. It is obvious in fighting. Building large structures is a way for society's dominator/s or perhaps society itself to posture in a "I'm bigger than you are" manner.
Examples of Dominance:
The Pharoahs and their pyramids. The Cathedrals were not so much the Catholic Church displaying dominance but individual towns trying to compete with each other to see who could build the biggest and tallest cathedral.
Easter island was another example. Those large statues, facing the sea, represented the ancestors of the island's inhabitants. They were built by competing clans who were trying to make their statue bigger than everyone elses. Then as they degraded their environment (The degradation was tied in with the statue making.) their society began to collapse and they started toppling each others statues.
The American manned Lunar mission. A direct response to Sputnik.
2. Embarking on long missions.
To fulfill the innate sense of Curiosity. Related to the general quest for knowledge and its ultimate manifestation, the scientific endeavor.
3. Remember, nature selects AGAINST waste and extravagance. Environmental pressures should have weeded out the "god gene" long ago, or so it would seem...
The 'god gene' is irrelevant and not a direct cause of civilization's hypertrophy. Regardless of how important religion was in ancient civilizations, it was simply something with which to imbue civilization with.
Your sense of "long ago" needs some adjustment. Remmber that Human civilization has grown exponentially in its (meaningless) extravagance in just 10,000 years. That is not too long ago when you consider that 'modern' Humans have momentarily buffered themselves from the consequences of environmental destruction.
This process of Human expansion is known as 'ecological release' in scientific terms. Ecological release occurs when one species is able to expand at the expense of other species due to lessened competition, for whatever reason.
With Homo Sapiens, this ecological release was the result of several factors.
a. It was pre conditioned by an advanced brain.
b. Under the right environmental circumstances, severe stresses accelerated technological advance.
The first environmental incident was the gigantic eruption of a volcano, about 70,000 years ago. It was powerful enough to have darkened the Northern Hemisphere for months and thus cause a major reduction of Hominid populations. That seems to have spurred, thousands of years after the eruption, previously small populations of Homo Sapiens that had been limited to Africa into a migratory expansion throughout the world.
In their expansion they eventually replaced other Hominid species.
The second environmental incident was the last ice age, which further accelerated technological innovation, particularly in Europe. Humans had reached Australia 40,000 years ago.
The third incident occured with the end of the last ice age. Humans could no longer maintain their hunter/gatherer ways for a variety of reasons related to population pressures. This led to the adoption of Agriculture which transformed Natural biomass into Human modified biomass. With Natural biomass, only a small fraction of it was edible by humans. With the Human modified biomass of Agriculture, a much larger percentage was edible by Humans.
This led to a population explosion and the conversion of most of nature into a human habitat.
Population has continued to expand greatly, leading to more destruction of the Environment.
Depending on how old you are, you may not have to hold your breath for too long to see Nature select against waste and extravagance on a massive scale. Two, three, four decades?
Villabolo
