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Estimating the times of origin and extinction of species

How did we get the information about the earliest appearance ("arrival") of the species as presented in Terminal Terra? There are two sources for such information: fossil evidence and DNA sequence comparisons. But both are difficult to interpret and provide at best only rough approximations of the actual dates of origin. For many if not most species it is impossible to find a documented estimate of the time of origin. In those cases the time of origin of the next higher group ("taxon") to which the species belongs and for which such information is available has been taken, i.e., the time of origin of the genus, family or order. Obviously, these datings are much older than that of the species. For example, the origin of Homo sapiens is placed at about 200,000 years ago [5], the genus Homo (also comprising e.g. H. erectus and H. habilis) at 1.9 Mya [6], the family Hominidae (also comprising chimpanzee, gorilla, orang-utan) at 13 Mya [7], and the order Primates at 78 Mya [7].

As for the extinction ("departure") of species, the information differs for fossilized and for more recently extinct species. For prehistorically extinct species, fossils are the only source of information. For species that became extinct in historical times, the extinction times are often reasonably well documented. When the last representative of a species died in captivity, the year of extinction is even precisely known, as for the North American traveller pigeon that died in 1914. But such cases are rare.

Fossils and their dating

The fossil record is highly fragmentary and incomplete, and for soft-bodied organisms almost non-existent. Fossilization is dependent on the presence of bones, shells or other hard parts, as well as on the habitat in which the species died. Marshes and estuaries, but also extremely dry or cold environments are well suited for fossilization. Groups like molluscs, mammals and many trees are therefore well represented in the fossil record, while fossils of birds, butterflies or worms are scarce.

Most methods for dating fossils are based on radioactive decay. A well-known dating technique is carbon-14 dating, which archaeologists prefer to use. The use of carbon 14 permits the determination of age directly from a fossil. However, the half-life of carbon-14 is only 5,730 years, so the method can only be used for "young" fossils, not older than about 60,000 years. For older fossils the age is found indirectly by determining the age of the associated rock by radiometric dating. This involves the use of isotope series, such as rubidium/strontium, thorium/lead, potassium/argon, or uranium/lead, all of which have very long half-lives, ranging from 0.7 to 48.6 billion years. Subtle differences in the relative proportions of the two isotopes can give good dates for rocks of any age [8].

Another approach for dating fossils is looking for surrounding index fossils. Those are fossils that are widely distributed around the earth, but limited in time span. Examples include brachiopods (which appeared in the Cambrian), trilobites (common throughout the Paleozoic), ammonites (from the Triassic and Jurassic periods), and many nanofossils (microscopic fossils from various eras which are widely distributed, abundant, and time-specific). Finally, also paleomagnetism can be used to estimate the age of a fossil, i.e., observations of the fluctuations of the Earth's magnetic field, which leaves different magnetic fields in rocks from different geological eras [9].

It should be emphasized that the dating of a fossil only tells us that a species existed at a given geological time point, but the actual times of origin and extinction may have been much earlier and later, respectively. Especially when fossils of a certain species are relatively rare, the actual existence-span may be much greater that the fossil record indicates.

Estimating species' origin from DNA sequences

DNA sequence comparison is obviously only applicable for extant species, and for some species that relatively recently became extinct and from which remains reasonably intact DNA can still be isolated. With recent technological innovations, "ancient DNA" studies are indeed increasingly feasible from samples of up to some 80,000 years old, such as from Neanderthals, mammoths and cave bears [10, 11]. Under ideal storage conditions the current record of long-term DNA survival is from Siberian permafrost (300 to 400,000 years [12]) and from deep ice cores in Greenland (450-800,000 years [13). Earlier claims of DNA sequences recovered from much older fossils have all turned out to be erroneous. Notorious examples are those from an amber-embedded beetle (120-135 million year old [14]) and from dinosaur bone (80 million year old [15]). Obtaining information about the origin of a species by means of DNA sequence comparisons is based on the principle of the "molecular clock, the fact that specific genes evolve at roughly constant rates. To derive the earliest possible time of origin of a species from DNA sequences one needs to determine the corresponding ("orthologous") sequences of the most closely related species. From these sequences a phylogenetic tree can be constructed, in which the sequences that are most similar are the nearest to each other. This tree then is considered to reflect the evolutionary relationships between the investigated species.

To give a time dimension to such a tree, one needs fossil datings for the earliest appearance of one or more of the species or higher taxa in the tree. Using those fossil datings as calibration, one can deduce the times of divergence of all other branching points in the tree. If species A and B are each others closest living relatives ("sister species") the time of their divergence is the absolute earliest date for the origin of both species. In fact, however, both species A and B only developed their distinguishing species-specific morphological features some time after their divergence. The actual time of origin of the species may thus be considerably more recent than indicated by the evolutionary divergence from their sister species. Moreover, molecular dating of species divergence points shows considerable variations, depending on the genes used, the included species, the fossil calibrations and the algorithms used for the calculations.

(See References for the numbered references above)