We are Jupiter's Heirs

IT has been known for some years that the gravity of the giant world of Jupiter protects Earth from cometary and asteroidal impacts––by capturing them as it circles the Sun. What is less well known is that Jupiter played a part in creating human civilisation. I do not mean that the sight of Jupiter inspired people to become civilised, I mean that but for Jupiter's ``existence'', the first great empire, that of the Pharaohs, might never have been born. This drama started in the Sahara, the largest desert on Earth that today gets only a fifth of an inch of rain each year. But 8,000 years ago the Sahara was a very different place. It was criss-crossed by

The Sahara was a different place - cross-crossed by rivers

rivers and had plentiful rain. Green, wooded and covered with lakes, it was the home of crocodiles, hippopotami, giraffes, elephants, gazelles –and people. This was no civilisation as we understand the term. Across a region of two million square miles were spread isolated villages of shepherds and herdsmen. Then, with comparative suddenness, the Sahara dried up. The rivers stopped flowing. The moist soil became sand, and the woodlands and animals vanished. Its human inhabitants were forced to migrate northwards to Egypt where conditions were still lush. There, restricted to a much smaller area and no longer free to wander, they began to build permanent structures. What did Jupiter have to do with the erection 5,000 years ago of the first pyramid at Sakkara by the grand vizier Imhotep? The answer is the periodic tilt in the Earth's axis caused about 6,000 years ago by the gravity of Jupiter and to a lesser extent Venus. The attitude of Earth towards the Sun changed by only half a degree, but it was enough to reduce the amount of sunlight reaching the Sahara to destroy its habitability. Egypt, hitherto uninhabitable because of the Nile's violent flooding, now became the only place in North Africa fit for living. This story has fascinating implications in the search for other civilisations in space. If an Earth-sized world lacks neighbouring planets that are close enough or massive enough to interfere with its axial tilt, nature may never force its intelligent inhabitants into small areas where they are compelled in turn to develop technology. Humans do not drastically change their way of life unless they are compelled to do so. Rural life in the vast and luxuriant Sahara must have been very pleasant. Who would willingly change it for urban rigour, slavery, and rule by fanatical priests? All species are similarly lethargic, and it seems reasonable to expect that all life-forms in the Universe will be the same. As telescopes continue to improve, there will come a time when we routinely discover alien planets the size of our own. To learn whether they are likely to harbour an intelligent civilisation, or a ``would-be'' intelligent civilisation, it will be necessary to examine their stellar systems closely. It will not be enough to ascertain whether their sun is stable and long-lived and whether their planet is at the right distance from it. It will be necessary to look at all those factors that could minutely influence its environment.