(And Harvard's Andrew Knoll has pointed out that it may have been impossible for "eucaryotes" -- cells with a nucleus -- to appear until the nitrate level in the oceans had risen to a certain level, which also required a high level of oxygen.

Since "procaryotes" -- non-nucleated cells such as bacteria -- apparently cannot form multi celled organisms, this means that oxygen was also indirectly necessary even for the appearance of multi celled plants.

Also, eucaryotes are much more vulnerable to solar UV light than procaryotes, and might have had to wait until Earth had built an ozone layer out of its new oxygen supply.)

But oxygen reacts very quickly with Earth's surface rocks and with the hydrogen and methane gases given off by its volcanoes; it could not appear at all in the air until a long time after "cyanobacteria" (photosynthetic bacteria, sometimes called "blue-green algae") had appeared and started producing it, and it would then have taken a long time to build up to levels that could support metazoans.

This in itself may explain why it took so long for metazoans to appear -- but it was by no means a sure thing for oxygen to build up in Earth's atmosphere, even after photosynthesis appeared.

When plants die and decay, after all, the very oxygen that they released into the air reacts with their tissues again and is re-absorbed -- so, for an excess of oxygen to build up in the air at all, a small part of the organic tissue from Earth's supply of dead plants must be sealed away from the air forever by being buried under sediment layers and trapped in shale before it can react with and re-absorb the oxygen that the plants had liberated while alive.

And an adequate rate of such burial is by no means a sure thing.

In a poster, Norman Sleep pointed out that for it to occur, new sediment must be poured into Earth's oceans by erosion at a fairly high rate -- and for that, Earth had to have unsubmerged continents whose rocks were weathered and eroded by wind and water and then swept by rivers into the sea.

Moreover, those sediments must be made mostly from granite -- for the sediment from basalt is rich in iron which itself eagerly reacts with the oxygen in the air.

So if Earth's total surface water supply were either too great (submerging most of its continents), or too small (reducing the amount of rain runoff, and also exposing the great basalt mountains of the mid-oceanic ridges), there would be much less oxygen in th air, and maybe none -- forever preventing the appearance of animals and maybe even multi celled plants.

And as David Catling pointed out in his poster, had Earth been too big, the same thing would have happened, because its exposed rocks and volcanic gases would have absorbed enough of the oxygen to keep it from building up to adequate levels for multi celled life for billions more years -- until the Sun became a red giant and roasted the planet.

Then there's the problem of climate (which I talked about in "SpaceDaily" last year).

To support any surface life or multi celled life, a planet must be in its star's Habitable Zone (or "HZ") -- not so close that a self-amplifying runaway greenhouse effect turns it into an inferno like Venus, and not so far away that self-amplifying runaway glaciation freezes all its surface water solid.

And since stars gradually increase their output with time, it must be able to remain in that zone for billions of years for any metazoans to evolve.

The HZ in our own Solar System seems to stretch from just a short distance closer to the Sun than Earth is (15 million km, at most) to considerably farther from the Sun than Mars (anywhere from 310 to 390 million km, depending on whether Forget and Pierrehumbert's theories about the warming effect of frozen carbon dioxide clouds are correct).

Within the HZ, another feedback effect actually tends to keep a planet's climate regulated.

When a planet gets warmer (unless it gets so warm that large amounts of its oceans turn into water vapor and further amplify its greenhouse effect, as happened to early Venus), the rate at which the carbon dioxide in its air reacts with liquid water and the planet's surface rocks to form carbonate minerals increases, in turn reducing the CO2 level and the greenhouse effect, and thus cooling the planet back down.

If it gets cooler (unless it gets so cool that its icecaps and clouds grow big enough and reflect enough sunlight back into space to further cool the planet), the rate at which this weathering effect pulls CO2 out of the air drops -- and since the planet's volcanoes are still belching CO2 into the air at an unchanged rate, the atmospheric CO2 builds up and rewarms the planet.

But even within the HZ, for this natural "silicate weathering" thermostat to keep the climate regulated, a planet has to have something else -- "crustal recycling".