The end of multicellular life (and of life itself) has been the subject of many documentaries, many of which are based on scientific observations and associated assumptions of the past in the rocks and out in space. Some are outright bizarre imaginings of people. But, one important aspect is that we need to look to the past evidence on Earth to gauge the likelihood of an event occurring in the future - though, given the fact that we are still exploring the mechanisms behind these past events, it is hard to say which may happen next.
The first place to look are the mechanisms of past mass-extinctions on Earth. A few examples are below:
An asteroid/comet impact - which is a widely accepted theory of the cause of theCretaceous-Neogene extinction eventthat wiped out a significant proportion of life 65 million years ago, including most of the dinosaurs. This potentially could occur at anytime with little or no warning.
Mass volcanism - such as the formation of the Siberian Traps, what is thought by many scientists to have caused the extensiveextinction event (over 90% life forms extinct) at the end of the Permian periodaround 250 million years ago. The onset of these may be detected.
Gamma ray burst - which some scientists hypothesise, through computer modelling, may caused amass extinction at the end of the Ordovician period, around 440 million years ago. This is very conjectural, but may occur without warning.
Nebula Winters, a relatively recent theory presented in the journalGondwana Research, in that encounters with astronomical nebulae can be (and according to theory in the article, has been) be catastrophic for life on the planet. Including their theory that a starburst may have caused the Snowball Earth global glaciation events.
Some of these events can be sudden, without warning, whereas some can occur concurrently and their effects take some time - for example, the extinction event at the end of the Permian is theorised to have had multiple causes - volcanism, impact, methane clathrate release etc and the extinction occurred in pulses separated by thousands of years.
Some suggest that we are in a mass extinction now, as presented in the article6th Mass Extinction? Humans Kill Species Faster Than They're Created, where human influences and manipulation may be replicating some aspects of past mass extinctions.