What could cause the human race to become extinct?

Earth's biological history has not been a gradual and leisurely saunter along evolution's varied and beautiful pathways. Instead, it has seen periods of biological growth and expansion punctuated with violent and brutal mass-extinction events.

Mass-extinction events are periods which, for whatever reason, see a sharp decline in the number of species on Earth in a relatively short amount of time. In Earth's biological history there is evidence of five major extinction events, which saw the demise of between 50 and 95% of all species present at the time of each event. These events (known colloquially as 'the Big Five'*) have been interspersed by smaller 'minor' extinction events which have each seen off smaller though still significant percentages of Earth's contemporary species.


The end of the world
Extinction events of different magnitudes have happened numerous times over Earth's history, and although studies of them appear to suggest that such events become less frequent over time, there is nothing to suggest that we've seen the last of them.

There are numerous theories as to what causes them, and each has to be researched in its own right, but some of the possibilities for previous extinction events include asteroid/comet impacts, prolonged, significant global warming and cooling, volcanism and many other possibilities.

The ones I'd like to concentrate on in this post are those relevant to astronomy- the big ones that are caused by events out of our control, and away from home.


What extraterrestrial accidents could finish us off?

Asteroid/ comet impact

This is the number one suspect for a number of previous extinction events, and there's nothing to say that it couldn't happen again. Depending on the size of the object that hits, we could be in for local destruction or near-complete eradication of life on Earth. Luckily, the likelihood of an impact decreases as the size of objects increases (simply because there are relatively few large objects and a lot of little ones), and the likelihood of anyone within our lifetime dying from such an event is very low indeed.

Alien invasion
I'm not actually talking about little green men descending from Mars and zapping us with their ray guns, here. Don't get me wrong: it's a possibility. But one thing that many people don't think about is the possibility of life in the form of bacteria or viruses being deposited on Earth by small asteroid impacts, and those organisms being incompatible with human life. No-one knows how likely this event is because so little is known about life anywhere other than Earth.

Local astronomical events
By 'local' I mean within a few light years, or possibly even further away, but within our region of the Milky Way. Events could include:

Artist's impression of a Gamma Ray Burst

• Gamma ray bursts: Caused by the collapse and supernova of a rapidly rotating, high-mass star, these are tight beams of intense radiation. If Earth was to be caught in one of these beams, it would definitely not be good news. Depending on the strength of the burst and the directness of the beam, effects could include the removal of the ozone layer, which absorbs much of the harmful radiation from the sun. Without the ozone layer, we'd probably be frazzled. Also, the gamma ray burst would flood the planet with radiation and energy of its own, causing widespread damage, destruction and death. Such an event is highly unlikely, but were the odds to come through there is nothing we could do to prevent it, and the destruction of life would be very high, if not total.
• Supernova: Caused by the collapse of a star, a local supernova could flood the solar system with radiation, causing the depletion of our ozone layer, with similar consequences as above. Again, not very likely, but also totally unpreventable should our number come up.

Even closer to home

• Solar flare: These are explosions in the atmosphere of the sun that release energy out into space. A large enough flare in the right (or wrong) direction could also strip the Earth of its ozone layer. A strong enough solar flare can result in a coronal mass ejection, which could cause further problems. Weaker solar flares are known to disrupt communications, and could potentially cause the collapse of power grids and other electronic equipment. Major loss of life is unlikely from most solar events, but damage would be widespread.
• Wandering black holes: It is possible that there are black holes left over from the early galaxy-forming days that wander the galaxy undetected. If one were to wander our way, it is very unlikely that we'd notice it until it devoured our planet. At least it wouldn't take long.

Far in the future...

• Death of the sun: When the sun dies, it will expand and Earth will be cooked to a crisp. There's a long time left before then, though, and the human race will most likely have either become extinct by some other means or moved on by then.
• Death of the galaxy: Everything in the galaxy will eventually either escape or be eaten by the massive black hole at the centre. Either way it's not good for us, and is entirely unpreventable, but it's so far in the future that we have plenty of time to prepare...
• Death of the universe: Eventually all matter in the universe will decay and, if we are living in a false vacuum as some scientists believe, this may eventually decay and cause chemistry as we know it, and therefore life, to become impossible. This event is not preventable, but so far in the future as to be incomprehensible.

Final thoughts

These events are all so unlikely as to be not worth worrying about, and almost not worth thinking about, but they are all possible, and are a part of thinking about the long-term future of the human race.

More information can probably obtained from this book**, written by Phil Plait, who writes the Bad Astronomy blog, which is well worth a look, however interested you are in astronomy***.




Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.

* The most recent is the 'Cretaceous- Tertiary Extinction Event', which happened about 65 million years ago, and said farewell to the dinosaurs.

** I would very much like a copy. My Amazon books wishlist is here!

*** He also writes quite often about other issues, including skepticism, anti-science and anti-vaccination issues.