One of the biggest problems that has puzzled researchers for generations is the question why almost all hominids, i.e. the 14 known species of human ancestors, have become extinct during the last 5 million years.

Ever since Charles Darwin, the predominant explanation has been that our ancestors were slowly and gradually replaced by more advanced species due to their inferior genes and their failure of adaptation, and that modern humans survived due to their superior "fitness".

In recent years, however, researchers have become aware that the fossil record does not show gradual but rather abrupt change. Neither the causes for the sudden gaps in the fossil record nor the underlying dynamics of hominid extinctions have been determined yet.

Dr Benny Peiser, a social anthropologist at Liverpool John Moores University and Michael Paine, an impact researcher from the Planetary Society in Australia, have come up with new findings that may help to solve this puzzle.

Peiser and Paine have calculated that the most likely cause of hominid extinctions may be associated with the more than 20 globally devastating catastrophes that occurred over the last 5 million years. Dr Peiser presented the findings last Thursday at the Charterhouse Conference 2001 "Celebrating Britain's Achievements in Space".

It is these large-scale impacts that had a catastrophic and detrimental affect on human evolution. A number of significant climatic downturns, well documented in the paleo-environmental and climatological records are directly associated with such major impact.

Episodes of cometary or asteroidal impacts punctuating human and societal evolution should thus be looked upon as natural agencies determining evolutionary regressions, extinctions and macro-mutations.

"The reason that Homo sapiens have survived in spite of these global disasters has little to do with the traditional explanations given by neo-Darwinists. It is sobering to realise that we are alive due to cosmic luck rather than our genetic makeup," says Dr Peiser.

"After all, the populations of hominids and early modern humans were extremely small. Had any of these impacts occurred in the proximity of these population groups, we might also have gone the way of the dodo."

Planetary Society's Michael Paine agrees: "Just over 2 million years ago an asteroid estimated to be 2km in diameter struck the Southern Ocean, south west of Chile. Had it struck land the environmental consequences might have been much worse. If the collision had occurred a few hours early southern Africa might have been wiped out, along with our ancestors."

Large impacts not only cause severe climate disruption (mainly darkness and cooling) but can also lead to the loss of the ozone layer (particularly with ocean impacts that propel chlorine into the upper atmosphere), severe acid rain and toxins.

"Such environmental mega-catastrophes may not only wipe out directly affected hominid species. What is more, the abrupt loss of the ozone layer and the sudden release of toxins may even affect the DNA in some unknown manner, thus triggering macro-mutations, including the sudden reorganisation of entire genomes," Dr Peiser suggests.

Given the rather conservative impact estimates, "it is almost certain that, for several years at least, creatures on Earth had to endure very severe conditions. Although the physical Earth heals quickly from impacts its inhabitants might not do so well and the course of evolution may well have changed due to these and similar impact events," says Michael Paine.

The findings are calculated on the basis of the generally accepted "impact rate" (i.e. the rate of cosmic impacts calculated from terrestrial and lunar impact craters together with the currently observable flux of asteroids and comets in the solar system).

A computer simulation of cosmic impacts over a 5 million year period was chosen to give an indication of the environmental disruption that have occurred during the evolution of our species.

These consequences can be categorised into:

A. Local - devastation over a radius of tens of kilometres. No serious regional or global consequences

B. Moderate regional - devastation over a radius of hundreds of kilometrees (the size of a small country). Short term regional climatic problems.

C. Severe regional - devastation over a thousand kilometres (the size of a large country). Severe regional climatic disruption. Mild short-term global climatic disruption (year without Summer).

D. Moderate global - devastation over thousands of kilometres (continents). Severe global climate disruption for several years. Global food chain failures

E. Severe global - global firestorms (ballistic re-entry of impact debris). extreme global climate disruption for decades to centuries. Extinctions.

The simulation looked at the "worst" event in each of 5000 millennia. It therefore gives an "underestimate" of the total number of impacts.

The program recognises 5 outcomes of an asteroid or comet colliding with the Earth:

• The object skims the atmosphere and flies harmlessly back into space. This happened in 2% of the millennia.

• The object explodes above land in an airburst similar to an atomic explosion. This happened in 17% of the millennia.

• The object impacts the land and forms a crater. This happened in 11% of the millennia.

• The object explodes in an airburst above an ocean. This was the most frequent outcome, accounting for 41% of millennia. Fortunately, until recently, most of these impacts would have been harmless to land dwelling creatures.

• The object impacts the ocean, forming tsunami and, possibly, ejecting vast quantities of water and salt into the atmosphere. This happened in 28% of the millennia. (Larger impacts may also reach the ocean bottom and cause similar effects to a land impact)

Size impactor (The letters refer to the typical environmental consequences.)

500 to 900m (C) - 108 events
1km to 1.5km (C/D) - 24 events
1.6km+ (D/E) - 13 events

Craters (The letters refer to the typical environmental consequences.)

Over the 5000 millennia a total of 552 craters were formed on land. Of these:

• 477 were less than 5km in diameter (A)
• 64 were between 5 and 10km in diameter (B)
• 9 were between 10 and 20 km in diameter (C) and
• 2 were more than 20km (D).

Therefore during this simulation severe climate disruption occurred, on average, every million years (2 land impacts and 3 ocean impacts).

The findings by Peiser and Paine are underpinned by a significant number of large impact craters. So far, 32 impact craters have been discovered that are younger than 5 million years. One is 52km in diameter, 3 are between 10 and 20 km, 1 is between 5 and 10km and 25 are less than 5km in diameter. However, it should be noted that it takes very unusual conditions to preserve craters of this size for more than a few hundred thousand years.

Related Links
Australian Spaceguard Studies
Liverpool John Moores University
SpaceDaily
Search SpaceDaily
Subscribe To SpaceDaily Express

Thanks for being here; We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain. With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords. Our news coverage takes time and effort to publish 365 days a year. If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.
SpaceDaily Contributor $5 Billed Once credit card or paypal		SpaceDaily Monthly Supporter $5 Billed Monthly paypal only