In the course of a normal day, the word "fossil" may quite possibly never occur to you. Fossils are, however, crucial to many aspects of daily life.

Coal, natural gas and oil (whose multifarious derivatives include many plastics), for instance, are not called "fossil fuels" for nothing -- each being the fossilized remnants of prehistoric plants without which modern life would swiftly grind to a halt. Concrete, too -- so beloved as a construction material in Japan despite its bland, boring grayness -- contains limestone formed from the fossilized remains of countless minute marine shelled creatures.

But fossils aren't all the stuff of prehistory, because there are "living fossils," too. These are life forms that disappeared from our fossil record at some point long ago, after which they were thought to have become extinct. Then, somewhere on Earth someone found a surviving example, or examples; and thus resurrected a "living fossil."

Among these, certainly the most headline-grabbing is the coelacanth, meaning "lobe-finned" fish. These first appeared about 400 million years ago, only to disappear from our fossil record about 65 million years ago, around the time we believe dinosaurs became extinct. Then, in 1937, a fisherman caught one in the Indian Ocean off Grand Comoro Island, from where others were also subsequently caught. Then, in 1998, a second species was found in waters off Sulawesi, Indonesia. Most recently, in November 2000, a third group, possibly of yet another species, was discovered off St. Lucia in KwaZulu Natal, South Africa.

Why that multimillion-year gap since the fossil trail went cold? Well, it seems that prehistoric coelacanths lived in environments conducive to the production of fossils. Their modern descendants, however, appear to survive only in submarine caves and in the dark depths beneath overhanging reefs -- two sites particularly unconducive to fossil formation.

Meanwhile, other less attention-grabbing living fossils have recently turned up on extinct underwater volcanoes between New Caledonia and Tasmania. Foremost among those found marooned on these isolated underwater "islands" have been species of primitive, air-breathing lungfish, which have now also been found in Africa and Australia.

Much closer to home, though, on any summer walk in the park or between station and home or office, there's a good chance you will benefit from the shade cast by another living fossil -- the ginkgo tree (Ginkgo biloba), known in Japanese as icho, and in English as the maidenhair tree.

Once unknown as a living plant, these trees with short, broad, fan-shaped leaves can now be found the length and breadth of Japan, from Kyushu to Hokkaido, and in temperate regions of Eurasia from Beijing to London, as well as across North America. Often planted in streets and parks, ginkgo survive well in such urban environments because of their natural resistance to city pollution and ability to thrive in relatively low sunlight.

This species, the only known survivor of the family Ginkgoaceae, was discovered in the Chekiang province of China in 1758, and has also recently been found growing wild in remote reaches of western China. Amazingly, comparing this living fossil with fossils from some 200 million years ago shows that this tree, which flourished along with the dinosaurs before the now-familiar plethora of flowering trees and grasses had even evolved, has hardly changed at all.

Its current abundance is in part due to it having been widely grown as a sacred tree in Chinese temple gardens since its continued existence came to light. It has also been prized throughout Asia for its medicinal properties. In addition, its hardiness and resistance to pollution has enabled it to spread globally.

Although superficially similar in appearance to other trees, the ginkgo is biologically remote from all other plants. Deciduous and capable of growing to a height of almost 40 meters, it is a slender tree with light gray-brown, grooved bark. In spring, bright yellow-green leaves emerge, appearing alternately along twigs and branches, or sometimes in dense clusters from coppiced or trimmed branches. The leaves are quite leathery and become a richer, darker green in summer before turning a cheerful bright yellow in autumn. The leaves (which are the official symbol of the Tokyo Metropolitan Government) have prominent veins arising from the base that split repeatedly in two. It is the resemblance of this venation to that of the maidenhair fern that gives the tree its English name.

Where the ginkgo begins to depart dramatically from the norm is in the reproductive department. Here, it is dioecious, meaning that each specimen is either male or female. The males grow thick, yellow catkinlike cones 6- to 8-cm long that shed pollen, while the females produce wind-pollinated flowers that resemble tiny acorns on long stalks.

The ginkgo is also a gymnosperm. Such plants -- including conifers, seed ferns and cycads -- produce naked seeds, bare ovules not contained in an ovary that, in other groups, later swells to become a fruit. The ginkgo, therefore, also doesn't produce true fruit, although females produce naked pale-green seeds that ripen in late autumn to yellow and then brown.

After fertilization, however, the thin seed-coating swells and becomes fleshy and fruitlike. This is loaded with butyric acid, which has an unpleasant, putrid smell some liken to rancid butter. Perhaps the smell originally evolved to deter predators from eating the seeds, but nowadays it also usually deters landscape gardeners from planting female trees.

Nonetheless, the large, foul-smelling white seeds -- called ginnan (ginkgo nuts) -- somehow came to be considered a delicacy in China, and in Japan they are served grilled or in hot egg custard.

Not only are ginkgo seeds prized as culinary delicacies, but extracts from ginkgo are also popular among herbalists.

Because ginkgo is a potent antioxidant and free-radical scavenger, it is believed to possess therapeutic powers that help increase stamina, improve circulation, increase longevity and counteract aspects of ageing such as degeneration of the retina and mental acuity.

Ginkgo extract also (allegedly) stimulates the libido. To think of all the fuss over Viagra, when a natural remedy awaits in the shape of this living fossil.