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A New Picture of Life's History on Earth

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A New Picture of Life's History on Earth Commentary A new picture of life’s history on Earth Mark Newman* Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501 hen most people think of paleon- his primary source (2, 3). Other compila- database is as exhaustive as possible, sub- Wtology, they picture the field pale- tions also have been published (4, 5), but stantial biases are introduced. For exam- ontologist digging up fossils with a tooth- Sepkoski’s database has received more ple, it is quite feasible that the increase in brush and publishing descriptions of the attention by far than any other. Sepkoski’s diversity toward recent times seen in Fig. anatomies of uncovered specimens in the database was simple in structure: it re- 1 is a result primarily of the greater vol- trade literature. New discoveries of dino- corded the first and last known occur- ume of rock available from recent times, saurs even make the national headlines. rences in the fossil record of more than and the greater amount of effort that has For many decades the discovery and de- 30,000 marine invertebrate genera in been put into studying these rocks. A scription of a previously unknown fossil about 4,000 families. Marine invertebrates number of studies over the years have species was the calling card that gained have been the focus of most statistical presented evidence showing that apparent would-be paleontologists professional ac- studies, because preservation is much diversity is closely correlated with the ceptance. However, in the last quarter of more reliable in marine environments and intensity with which different periods of a century or so, many of the most intrigu- invertebrates are much more numerous geologic time have been sampled (6, 7). ing new results in paleontology have come than vertebrates. The new database not from field studies, but from the com- Time was mea- compiled by Alroy pilation and analysis of large-scale data- sured in strati- and coworkers (one bases of fossil species. These databases graphic stages, un- John Alroy, Charles Marshall, and a of whom is the same have provided us with quantitative pic- even intervals large group of distinguished Jack Sepkoski men- tures of the pattern and size of mass defined by using a tioned above) at- collaborators report on the extinction events, the rate at which new variety of geologi- tempts to correct species have appeared, and crucially the cal and paleonto- creation of a new database some of these prob- number of species on the planet through logical markers. that catalogs fossils at the level lems by including time, the so-called standing diversity. In Many features of more comprehensive an article appearing in this issue of PNAS the fossil record of individual collections. data about fossil COMMENTARY (1), John Alroy, Charles Marshall, and a have been de- taxa, in particular di- large group of distinguished collaborators duced from Sep- viding data into col- report on the creation of a new database koski’s data. One of the most famous is lections—groups of fossils recovered from that catalogs fossils at the level of individ- shown in Fig. 1, which is a plot of the total specific locales by specific workers or ual collections. Preliminary analysis of this number of genera in the database as a teams—with repeated occurrences of taxa database reveals interesting results, call- function of time during the Phanero- at different times and places explicitly noted. ing into question some fundamental ideas zoic—approximately the last 540 million Like the database of Sepkoski, the new about the history of life on Earth. years, from the so-called ‘‘Cambrian ex- database focuses on marine invertebrates, There are three principal features wor- plosion’’ of metazoan diversity until the and is at present incomplete—work is still thy of note in the paper by Alroy et al. present day. The shape of this curve mir- continuing on the compilation. Currently it First, the paper announces the creation of rors the accepted view of life’s history on covers two time periods of about 150 million the new database. Second, the authors the planet: a burst of diversification in the years each, one in the middle part of the describe new methods of data analysis Cambrian and Ordovician, followed by a Paleozoic, during the plateau seen in Fig. 1, made possible by the database that help to rough plateau in diversity for about 200 and one from the mid-Mesozoic to the eliminate biases inherent in previous stud- million years in the latter half of the Paleo- mid-Cenozoic, the central portion of the ies as a result of variations in patterns of zoic, until the dip in the center of the figure, diversity increase in the right-hand part of fossil preservation and collection. Third, which represents the massive late-Permian the figure. these new methods raise doubts about the extinction event. Following this extinction, it Because of the division of the database long-held belief that biodiversity has in- appears that diversity first recovered and into collections, Alroy and coworkers creased dramatically in the last 250 million then increased substantially during the have been able to compensate for biases in years; it may in fact be that diversity has Mezozoic and Cenozoic, rising to a present- the intensity of sampling of different time been roughly constant, although no firm day level two or more times higher than any intervals, and to some extent for varying verdict has been reached yet on this point. seen during the Paleozoic. quality of fossil preservation in their data, Statistical analyses of species turnover Sepkoski’s database, although extensive and so make more accurate estimates of and diversity in the fossil record have been and thorough, has a number of shortcom- diversity (although, as they are the first to dominated in the past by the work of one ings. In particular, it records only first and emphasize, biases are still present). Their man, Jack Sepkoski, who from the early last occurrences of taxa anywhere in the technique of analysis involves breaking eighties until his death in May 1999 world, and no other data, such as how the data down in two ways. First, they worked single-handedly on the compila- commonly taxa occur or where. Thus very tion of an encyclopedic database of oc- widely occurring taxa are accorded exactly currences of marine invertebrates in the the same status as ones that are found See companion article on page 6261. fossil record, using journal publications as rarely. Also, by the very fact that the *E-mail: [email protected] www.pnas.org͞cgi͞doi͞10.1073͞pnas.121190498 PNAS ͉ May 22, 2001 ͉ vol. 98 ͉ no. 11 ͉ 5955–5956 Downloaded by guest on September 27, 2021 intervals that have the same total count can be expected to have approximately equal real diversities. That is, the results are comparable between different geo- logic times. To some extent, the principal new con- tributions of the present study are the database itself and the sampling-standard- ized methods for measuring diversity. However, the preliminary results also of- fer some interesting suggestions of what is to come in this field. The authors make a host of different observations about the results of their calculations, but perhaps the most interesting is that most of their measures of biodiversity are found to give approximately equal figures for diversity in the two time periods studied. Recall that in the curve of Fig. 1, derived from the earlier work of Sepkoski, the two periods showed very different behavior, the first having a rough plateau in diversity, the second showing a marked diversity in- Fig. 1. Total number of genera as a function of time during the Phanerozoic, in the classic database of crease. This increase is not clearly visible Sepkoski (2, 3). The two dark blue regions indicate the time intervals studied in the new work by Alroy in the new results, suggesting that the et al. (1). supposed post-Paleozoic diversification of marine fauna may be merely an artifact of biases in the Sepkoski database. It should divide the data into roughly equal time from one collection to another. This, how- be emphasized however, that these results intervals—more uniform in length than ever, may not be the case, so they also use are by no means final, and it is too early to the intervals used by Sepkoski. Second, several other techniques that weight lists draw any firm conclusions from the data. within each interval they attempt to according to their length, and they report The creation of this new database of the choose a constant number of actual fossil separate results for each of the different fossil record may well have far-reaching specimens, as if the intensity of sampling methods used. Clearly in the absence of effects. The mere fact that most of the across different times and places had been more detailed information about which previous work in this area has made use of uniform, rather than widely varying as it in weighting is correct, only results that are just a single source of data—the Sepkoski compilation—makes the creation of an fact is. Unfortunately, only the number of robust across different methods should be independent database an important and taxa is recorded for many of their collec- considered to have strong support. worthwhile enterprise. However, the in- tions and not the number of specimens, so The diversity counts given by Alroy et al. clusion in this new database of far more it is not possible to fix specimen number are the total numbers of taxa seen across detailed information on frequency of oc- directly.
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