Sources of Biodiversity Record

Knowledge of the history of biodiversity is derives from two primary sources. The first is analyses of data from the fossil record, and the second is analyses of molecular data.

Fossil Record

Much of the modern-day geological landscape owes its origins to past biodiversity, which has left behind a rich fossil record. This has provided extraordinary insights into the history of life on Earth.

However, working with the fossil record to understand this history is an important constraint for three reasons. First, as recognized by Darwin when marshalling evidence for his theory of evolution, this record is far from perfect or even.

The record is much better for some periods than for others, and estimates of the numbers of species leaving a fossil record range from less than one to, at most, a few per cent of those that have ever lived.

Second, of this fossil record, only a tiny fraction has actually been recovered. Third, the record, and that portion of it that has been recovered, is biased towards the more abundant, the more widespread, and the longer lived species, and more towards some groups of organisms than others.

For instance, soft-bodied organisms, such as some cnidarians (jellyfish, sea anemones) are rarely fossilized and are exceptional in the fossil record, whereas the number of individual fossils of brachiopods, which are hard-bodied organisms, has been estimated to be in the billions.

Some of the major soft-bodied animal groupings have left no fossil remains: animals like the Platyhelminthes (flatworms, flukes and tapeworms).

The fossil record for animals with hard body parts, such as the brachiopods and molluscs, echinoderms and vertebrates, while often much better, is still far from complete and not always representative: 95% of all fossil species are marine animals while 85% of today’s recorded plants and animals are terrestrial.

In short, many of the pages of the history of biodiversity written in the fossil record are missing, and those that have been obtained only capture a biased portion of that history.

The paucity of the fossil record, even with regard to individual taxa, is well illustrated by a group that possesses hard body parts and is relatively well researched, having caught the attention and imagination of people of all ages and from all walks of life: the dinosaurs.

Although something of the history of this group is familiar even to many primary / elementary school children, it remains based on a remarkably small window on the past. As of 1990, 900–1200 genera of dinosaurs were estimated to ever have lived.

Of these, only 285 (336 species) were known from fossils, and nearly half of these were from only a single specimen; complete skulls and skeletons were known from only 20% of known genera. Similarly, it has been estimated that no more than 7% of all the primate species that have existed are known from fossils.

While it is clear that the documented fossil record is far from complete, in many different ways, it still provides an invaluable pictorial history of life on Earth, where many of the major events in that history have left their mark in, or on, the rocks.

Notwithstanding its limitations, it is still possible to construct an understanding of changes in biodiversity through geological time using the fossil record.

However, because of the constraints, it will often be necessary to make recourse to the temporal dynamics of numbers of higher taxa rather than of species because these are less vulnerable to the constraints.

This should not pose too much of a problem, for not only do numbers of higher taxa act as a surrogate for numbers of species but it is also true that they act as a measure of biodiversity in their own right.

Molecular evidence

Whilst the fossil record continues to provide the bulk of insights into the history of biodiversity, molecular evidence is playing an increasingly significant role.

Comparison of molecular data for different organisms enables the generation of branching trees representing hypotheses of their patterns of phylogenetic relatedness, with those organisms with sequences that are more different being assumed to have diverged earlier in the evolutionary process.

If assumptions are made about the rate at which molecular sequences diverge (a ‘molecular clock’), then the timings of different evolutionary events can be estimated.

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Fossil and molecular evidence do not always agree, particularly over the dates of first appearance of groups.

For example, molecular evidence suggests that at least six animal phyla originated deep in the Precambrian, more than 400 million years (Myr) earlier than their first appearance known from the fossil record.

Likewise, molecular data suggest that primates diverged from other placental mammalsc. Myr ago whereas the oldest known fossil primates are from 55Myr ago. The fossil record is always liable to underestimate dates of first appearance, because the likelihood of such early individuals being fossilized and the fossils recovered is low.

Equally, of course, the accuracy of first appearances estimated from molecular evidence rests on the interpretation of the molecular divergence data and particularly on the assumptions about the nature and dynamics of the molecular clock. However, together, fossil and molecular evidence provide a powerful combination for unlocking many of the secrets of the past.

In conclusion, tracing the reason for the diverse and varied organisms will be practically impossible using just the phenotypic characteristics. Detailed evidence is derived from analysis of fossil data and subjecting to samples to molecular examination.

Knowledge of the history of biodiversity is derives from two primary sources; analyses of data from the fossil record, and analyses of molecular data. Fossils are important in the study of the origin of past biodiversity, however there are pitfalls. While it is clear that the documented fossil record is far from complete, in many different ways, it still provides an invaluable pictorial history of life on Earth; where many of the major events in history have left their mark in, or on, the rocks.

Notwithstanding its limitations, it is still possible to construct an understanding of changes in biodiversity through geological time using the fossil record.

However, due to the constraints, it will often be necessary to make recourse to the temporal dynamics of numbers of higher taxa rather than of species because these are less vulnerable to the constraints.

While fossil record continues to provide the bulk of insights into the history of biodiversity, molecular evidence is playing an increasingly significant role.

Comparison of molecular data for different organisms enables the generation of branching trees representing hypotheses of their patterns of phylogenetic relatedness, with those organisms with sequences that are more different being assumed to have diverged earlier in the evolutionary process.

The fossil record is always liable to underestimate dates of first appearance, because the likelihood of such early individuals being fossilized and the fossils recovered is low.

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Equally, of course, the accuracy of first appearances estimated from molecular evidence rests on the interpretation of the molecular divergence data and particularly on the assumptions about the nature and dynamics of the molecular clock.

However, fossil and molecular evidence together provide a powerful combination for unlocking many of the secrets of the past.