The fossil record
But is this what really happened? At first glance, it seems as if it shouldn't be too difficult to demonstrate one way or the other. You simply turn to the fossil record and look for evidence of how life evolved over vast stretches of time. Here, just as in archaeology, you can examine the footprints of ancient history, and piece together a jigsaw picture of strange, long-forgotten eras during nature's march towards the present.
Darwin, like every other evolutionist before and since, turned repeatedly to the fossils to give him support for his theory. Naturally, he hoped to find his ‘finely graduated organic chain’ showing the many transitional forms that led up to the giraffes, jellyfish, etc., that we see today.
Anyone who has puzzled about how fossilized sea-shells come to be on the tops of mountains, or piled hundreds of feet high to form such landmarks as the white cliffs of Dover, must realize how difficult it has been for geologists to place the fossil record in some kind of rational order. Because of the upheavals brought about by continental drift, huge changes have taken place in Earth’s landscape, and the rock strata are convoluted in the extreme.
Nevertheless, geologists feel with some confidence that their Modern maps are accurate. Even though there is nowhere in the world you can find an unbroken succession of fossils and rocks from the beginning of time until now, it has been possible to correlate fossils from different places and arrive at a fairly consistent history of Earth's evolution. As we shall see later, a new approach to comparative anatomy and the relationship of fossils to each other, developed during the 1970s, has thrown considerable doubt on important aspects of this history. The familiar textbook trees of life may be altogether too complacently drawn.
However, almost everyone still agrees with the broad picture that emerges from the fossil record. Life has evolved, with occasional interruptions, in ever-growing variety and complexity. There have been a number of fairly well-defined periods when certain life forms have dominated the Earth, replaced eventually by more modern forms. And the main evolutionary developments in the animal world have been successively:
- Minute, single-celled organisms (bacteria and slime).
- Multi-celled invertebrates (e.g. sponges, sea-food, jellyfish).
- Fishes with backbones.
- Amphibians living partly on land.
- Reptiles (including dinosaurs).
- Mammals and birds.
The fossil record readily shows this succession of forms, and Darwin hoped it would do more. He believed, and it has remained the conventional wisdom, that these stages were linked. Fish turned into amphibians, amphibians turned into reptiles, reptiles turned into birds, and so on.
Moreover, Darwin was convinced that this process happened bit by bit - minute ‘improvements’ in successive generations gradually led to the emergence of new species. He took this stance in spite of warnings from his friends - for instance, Thomas Huxley, who wrote him a letter on the day The Origin of Species was published. Darwin was taking a risk, he said, in becoming inextricably married to the idea of slow and constant evolution as summed up in the Latin tag Natura non facit saltum (Nature does not make leaps). 'You have loaded yourself with an unnecessary difficulty,' he cautioned.
But Darwin was adamant, and Huxley's warning notwithstanding, most museums and textbooks today accept gradualism as readily as they accept natural selection. Indeed, the two are inseparable. The touchstone of neo-Darwinian theory is that evolution results from the natural selection of small, accidental, cumulative changes.
Logically, then, the fossil record ought to show this stately progression. If we find fossils at all, and if Darwin's theory was right, we can predict what the rocks should contain: finely graduated fossils leading from one group of creatures to another group of creatures at a higher level of complexity. The minor 'improvements' in successive generations should be as readily preserved as the species themselves.
But this isn't so. In fact the opposite is the case, as Darwin himself complained: Innumerable transitional forms must have existed but why do we not find them embedded in countless numbers in the crust of the earth?'
