December 30, 2008, 10:00 pm Olivia Judson Wild Side Blog

An oyster shell sits on the table in front of me. But I’m not about to have an oyster feast: I won’t be squeezing a lemon, grinding pepper and lifting the shell to my lips. This isn’t because I’d choke, though the animal from this shell would be far larger than one throatful. It’s because this oyster died more than 20 million years ago, and the shell is empty.

It has none of the beauty of a modern shell. Inside, the lining has changed from pearly iridescence to smooth, gray stone. The outside is free from the weeds and dirt that cover a living shell, and the colorful markings that would once have adorned it have gone. There is no smell of the sea. Indeed, the seabed where this animal once lived is land now.

or a fossil, 20 million years is young. The oldest animal fossils are around 570 million years old; the last dinosaurs vanished 65 million years ago. But 20 million years is still long enough for the world to have looked noticeably different from the way it does today. When this oyster was alive, the Himalayas and the Rocky Mountains were just starting to rise up; the Grand Canyon had not yet formed. South America — where this oyster’s from — was an island.

As I run my hands over the rough crenellations of the shell’s outside, as I feel the weight of the stone in my hands (and it is heavy — 1.3 kgs, or nearly 3 lbs.), I can’t help feeling a kind of reverence for this fragment of the fabric of the past.

It’s hard to become a fossil, to leave a tangible record of your presence on the Earth millions of years after you died. Most of us swiftly get recycled into other beings. After all, the competition for corpses is fierce. Species of bacteria, worms, ants, flies, beetles and even some butterflies have a taste for rotting flesh. And that’s without mentioning larger scavengers, like vultures, hyenas and mongooses.

The disappearance of a body can be rapid. To give one of my favorite examples, in the tropical forests of the Congo, an adult male gorilla — all 150 kg (330 lbs.) of him — will be reduced to a pile of bones and hair within 10 days of his death. Within three weeks, there will be nothing left but a few small bones. And this is without the help of creatures like hyenas, which pulverize and eat the bones of all but the largest animals. (That’s why hyena scat is white: it’s the remains of powdered bone.)

But evading Nature’s undertakers is only the first step in becoming a fossil. If you want to be preserved for millions of years, you also have to choose the right place to die. If you’re lucky, you’ll have a quick burial in, say, the silts and sediments of a river bed, or under volcanic ash.

For many environments can never yield fossils. Die on top of a mountain, for example, and your fossil hopes are slim. The reason is that mountains don’t bury, they erode. (You might get frozen in ice, in which case you may last as long as the ice does, which may be several thousand years; but it won’t be several million. Ice is for those with modest ambitions for immortality.) Likewise, if soil is too acidic, bones dissolve. That’s why forest animals leave few fossils: forest soil tends to be acidic.

Even if you manage to die in the right place, you’ll have a better chance of surviving in death if you have the sort of body that can leave hard remains. In the fossilization stakes, animals with shells — like oysters — have an advantage over those without (jellyfish, say).

All this means that the fossil record of the Earth is inherently skewed. For instance, river deltas are great places to get buried and preserved. So animals that lived in or near them are much more likely to make it into the fossil record than most other creatures; as a result, we have river-delta fossils in much greater numbers than most other types. But during life, those animals were by no means the most numerous. As one friend put it, it’s like making an inventory of current North American wildlife based on what you find at the mouth of the Mississippi.

In light of this, the fossil record we do have becomes the more amazing. Yes, it has limitations. Yes, there are many organisms that we can never know about, for we will never know they existed. They breathed, and changed the atmosphere; they preyed on other beings; their carcasses became food, and altered the composition of the soil; but they left no physical trace, no clues to what they looked like, to the lives they led, the mates they seduced, the songs they sang.

Yet it is not surprising that the fossil record is incomplete — how could it be otherwise? What is remarkable is that we know as much as we do about the lives of the organisms of the past. The sciences of taphonomy — how bodies decompose and eventually become stone — and paleontology have allowed us to piece together many details of ancient ecosystems. And recent years have yielded up an astonishing wealth of “transitional forms” — organisms with bodies that are in between those of, say, dinosaurs and birds, fish and amphibians, or even whales and their nearest living relations, the hippos.

But as I sit here at the threshold of the new year, contemplating this oyster, what strikes me most powerfully is that the impact of ancient