In order to comprehend why Life has spawned a tool-building species, we first have to embed the human story within the Earth’s biography.
The Earth was formed by the accretion of the solar nebula about 4.5 billion years ago. While liquid water condensed on the surface of the Earth soon after, the first life forms on Earth appeared 3.5 to 3.8 billion years ago. But these were simple single-celled organisms. Complex ecosystems containing plants, animals and insects did not form until tens of millions of years after the Cambrian explosion occurred about 540 million years ago. Since that explosion, the biodiversity of species has been increasing almost monotonically and lately, almost exponentially, but interspersed with minor and major extinction events sprinkled throughout the fossil record. Of these extinction events, five major extinction events were identified in a landmark paper in 1982:
1) The Ordovician-Silurian extinction event: 450-440 Million years ago (Ma) at the end of the Ordovician period. It is speculated that two separate events occurred that killed off 60-70% of all species that existed at that time.
2) The Late Devonian extinction event: 375-360 Ma at the end of the Devonian period. It is estimated that this extinction event occurred over a prolonged period of about 20 Million years, with as much as 70% of all species dying out in that interval.
3) The Permian Triassic extinction event: 251Ma at the end of the Permian era. This is known as the Great Dying with as much as 90% of all species dying out. There is evidence that the atmosphere was full of hydrogen sulfide at that time, most likely caused when a climactic warming upset the oceanic balance between deep water sulfate reducing bacteria and photosynthesizing plankton. Hydrogen sulfide is not only poisonous to both marine life and life on land, it also weakened the ozone layer exposing life-forms to UV radiation as well.
4) The Triassic-Jurassic extinction event: 200Ma at the end of the Triassic era. It is estimated that 70-75% of all species went extinct and this event led to the rise of the dinosaurs.
5) The Cretaceous-Paleogene extinction event: 66Ma at the end of the Cretaceous era. This event led to the extinction of all non-avian dinosaurs and the rise of the mammals. The prevalent theory is that numerous celestial objects, mostly asteroids, bombarded the Earth over a 300,000 year period and this resulted in the extinction event.
While these extinction events stand out, there are tens of other mass extinction events in the paleontological record and there are numerous hypotheses on potential causes for these extinction events. Identifying specific causes for particular mass extinction events is one of the most interesting detective tasks in the field of paleontology. The proposed cause should explain why the specific species groups died out while others survived the event and should be based on corroborating evidence that the conjectured events actually happened. In general, most mass extinction events can be traced to sudden climactic changes, either a sudden warming or a sudden cooling of the Earth's global surface temperature. The most commonly suggested causes of these sudden climactic spikes are as follows:
1) Sustained volcanic eruptions: Sustained volcanic eruptions produce dust and sulphur dioxide (SO2) emissions that block sunlight and therefore, impede photosynthesis and cause a cooling spike in the Earth's global surface temperature. The SO2 rains as sulphuric acid after reacting with water vapor in the atmosphere, killing vegetation. Furthermore, sustained volcanic eruptions build up CO2 in the atmosphere which means that once the eruptions stop, the dust settles and the SO2 aerosols are removed from the atmosphere within a few days, the built-up CO2 which lasts in the atmosphere for hundreds of years, causes a sudden increase in the Earth's global temperature resulting in the mass extinction event.
2) Impact events: The impact of a sequence of sufficiently large celestial objects, like the ones that famously killed off the dinosaurs 66 Ma, could have similar consequences as sustained volcanic eruptions. Such impacts could result in global wildfires, acid rain and global heating due to large CO2 build-up in the atmosphere. It is believed that a large asteroid impacting the ocean could have worse consequences for life than one impacting land, since a local heating of seawater over 50oC due to such an impact can result in a huge spike in CO2 emissions from the ocean.
3) Clathrate Gun: A warming of the Earth's global surface temperature can thaw large amounts of frozen methane (methane that is stored in a lattice of ice) that is normally found in the Earth's colder surfaces. Since methane is a much more powerful greenhouse gas than CO2, this can become an amplifying feedback since the methane emissions would increase the temperatures further.
4) Gamma ray bursts: A nearby supernova or gamma ray burst in the galaxy, within 6000 light years of the solar system, could cause the ozone layer to be weakened exposing life to fatal levels of UV radiation. But gamma ray bursts are rare, occurring only a few times in the Milky Way galaxy every million years, though it is suggested that a gamma ray burst was one of the events responsible for the End Ordovician extinction that occurred 450-440 Ma, just as complex life was beginning to evolve on Earth.
Other than the fourth type of event, which is extremely rare, the effects of the other events are similar to what is occurring on Earth today due to human industrial activities. Humans have deliberately pumped greenhouse gases and aerosols into the atmosphere at a pace comparable to what would have occurred if a sustained volcanic eruption had happened. Humanity is being forced to perfect methodologies that would draw down these atmospheric gases on Earth reliably, on an ongoing basis. Also, in the process of becoming a global industrial civilization, humans have developed powerful telescopes, space travel technologies and nuclear weapons technologies that make it possible to combat any barrage of large asteroids, comets and meteors from ever impacting the Earth again.
Therefore, I contend that Mother Earth's purpose of spawning of a superlative tool-building species was to combat the primary causes of major extinction events that had happened on Earth! It's as if Mother Earth inoculated herself with a vaccine, our species. True, the vaccine has caused a mild fever and a mild extinction event that is gathering momentum, just as a medical vaccine in a human being is prone to cause mild symptoms of the disease that she's being inoculated against. But both of these symptoms can be ameliorated if humans begin taking the right decisions now, once we seriously assume our responsibilities in our ecological niche within the planet's ecosystems - as the “sentinel caregivers,” the “Khalifahs” for all Life. As we assume this role in our Butterfly phase, we would be like the prairie dog sentinel in the prairie dog colony that is the Earth's biosphere.
Imagine the following news item, dated Sep. 8, 2015:
"Discovered on August 24, 2015, by the Lincoln Near Earth Asteroid Research Project, an MIT Lincoln Laboratory program funded jointly by NASA and the US Air Force, in New Mexico, the asteroid code named 2014 QQ47 has been classified as a 10 on a Torino scale of impact hazards, the highest threat level possible. The orbit of this asteroid has been calculated using 51 observations over a 2 week period and scientists are 95% certain that it is headed for a collision with the Earth on March 21, 2026. If this asteroid strikes the Earth intact, the impact will have the effect of over 80 million Hiroshima style atomic bombs. As Billy Bob Thornton says in Armageddon, “It's what we call a Global Killer....the end of mankind. Half the world will be incinerated by the heat blast.....the rest will freeze to death in a nuclear winter. Basically, the worst part of the Bible!”
The White House issued a press release stating that President Obama is in emergency meetings with NASA scientists, military officials and world leaders to devise a strategic response to the asteroid threat. He urged the American people and the people of the world to stay calm and rest assured that the world's military powers have the technological capabilities to defend the Earth from this cataclysmic threat.
Scientists from the NASA Jet Propulsion Laboratory in Pasadena, California, stated that the sooner we begin a crash space program to transport and detonate a large thermonuclear device on the asteroid, the better. But even if such a detonation is successful, there is a strong likelihood that tens of thousands of large asteroid debris would rain on Earth on March 21, 2026 possibly requiring an unprecedented effort involving thousands of military personnel tracking down and shooting the larger debris with missiles before they impact the Earth."
Just imagine the unprecedented cooperation among people and nations that would occur when faced with such an external, existential threat. Gone would be all our differences over race, color, creed, national boundaries, sexual orientation or whatever utter triviality that divides us these days. Gone would be our self-doubt over our preoccupation with technology, especially military technology, over the past 200 years. We would even be so universally thankful that Americans pioneered the development of nuclear weapons and space travel technologies over the past century. There is no doubt that we will come together as one humanity to respond, to protect the Earth, not just for our sake, but for the sake of all our fellow Earthlings. We would then have discovered our true identity as a species, as a sentinel caregiver for all Life on Earth.
This is not such a fanciful threat, either. The first paragraph in the above imaginary news item appeared almost verbatim in a news item in 2003, except that I changed the dates by 12 years and I changed the threat level on the Torino scale from 1 to 10. Subsequent observations and refinement of orbit calculations of the asteroid 2003 QQ47 downgraded the threat level on the Torino scale to 0, a false alarm.
In the same year, 2003, a scientific paper published in the Proceedings of the American Institute of Physics settled the question:
"Preventing collisions with the Earth by hypervelocity asteroids, meteoroids, and comets is the most important immediate space challenge facing human civilization. This is the Impact Imperative. We now believe that while there are about 2000 earth orbit crossing rocks greater than 1 kilometer in diameter, there may be as many as 200,000 or more objects in the 100 m size range. Can anything be done about this fundamental existence question facing our civilization? The answer is a resounding yes! By using an intelligent combination of Earth and space based sensors coupled with an infra‐structure of high‐energy laser stations and other secondary mitigation options, we can deflect inbound asteroids, meteoroids, and comets and prevent them from striking the Earth. This can be accomplished by irradiating the surface of an inbound rock with sufficiently intense pulses so that ablation occurs. This ablation acts as a small rocket incrementally changing the shape of the rock’s orbit around the Sun. One‐kilometer size rocks can be moved sufficiently in about a month while smaller rocks may be moved in a shorter time span."
But why did the Earth spawn such a sentinel caregiver species now? Clearly, there have been numerous extinction events in the past 500 Million years of the Earth's history, during which complex life evolved and indeed, biodiversity thrived as a result of these extinction events. Therefore, what makes this era so critical to demand the presence of such a species? To understand why the Earth deems this so critical, we need to consider the evolution of the sun and the history of atmospheric CO2 on Earth.
 Raup, D.; Sepkoski Jr, J. "Mass extinctions in the marine fossil record". Science 215 (4539): 1501–1503. 1982. doi:10.1126/science.215.4539.1501 http://bit.ly/1XFYfqg
 Here is an article from 2003 on QQ47: http://bit.ly/25egxEF
 Here is an article from 2003 downgrading QQ47: http://bit.ly/1svJaeE
 Campbell, et al, “The Impact Imperative: Laser Ablation for Deflecting Asteroids, Meteoroids, and Comets from Impacting the Earth,” AIP Conf. Proc. 664, 509 (2003); http://dx.doi.org/10.1063/1.1582138