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lion species, has developed in around 4,000 million years on the basis of the ability of life forms to adapt and multiply at a rate that surpassed extinctions. Species, including humans, depend on the ecosystems that have operated with no or minimal human intervention up to recent years. Recent extinction rates are 100–1,000 times their prehuman levels in taxonomically diverse groups from widely different environments. Moreover, it is accepted that if all species currently endangered become extinct, then future extinction rates will be 10 times recent rates (1). Although chemicals are the basic units for the development of life, it seems meaningful to take into account Paracelsus’ statement that “all things are poison and nothing is without poison.” From this perspective it seems obvious that chemical and physical features have been considered driving forces of evolutionary processes from the beginning of recorded history. Estimates suggest that the current world production of chemicals is 400 million metric tons. Almost 11 million naturally occurring or man-made chemicals have been identified in the CAS Registry File, although only a small portion of them is commercially available. I would like to present the hypothesis of a direct link between chemical stress and a major mass extinction process, the Cretaceous-Tertiary (C-T) event, with the aim of providing a more holistic view on the potential of chemical stress on the evolutionary process. The earliest evidence of stable continents and oceans and records thought to implicate primitive life seems to point to the late Hadean (the Dark Ages)–early Precambrian era, about 4 billion years ago. Microorganisms have flourished in the oceans since at least 3.8 billion years ago, and based on the occurrence, elemental ratios (carbon, hydrogen, nitrogen, phosphorus) and isotopic compositions of this organic matter and its host rocks indicate that microbial mats developed on the soil surface around 2.7 billion years ago. Put simply, life is a property of certain configurations of matter, an emergent phenomenon that eventually can be passed on from generation to generation and can modify itself in order to persist on longer timescales, as the environment changes. Although only a small fraction of all the potential evolutionary possibilities developed, life forms were successful enough to inhabit and even transform the Earth into a living planet—the Gaia hypothesis based on the stability of self-regulating processes such as the Earth’s atmosphere during millions of years in spite of chemical disequilibrium. Although the sampling intensity from time to time and place to place could bias our knowledge on the existant biodiversity during different periods of the Phanerozoic Era, the fossil record of nonmarine tetrapods from the Devonian era (approximately 400 million years ago) onward exhibits an average of about 50 families until the last quarter of the Cretaceous period, when the number of families increased to nearly 80; the rise continued to the present 337 families in several steps in the Tertiary period (2). A similar pattern of increase in the number of families was also found in the record of marine invertebrates (3). Conversely, extinction throughout earth history is generally estimated to include more than 90–99% of the species that ever lived. Thus, extinction is the rule rather than the exception, and the vast majority of these extinctions are considered to be normal or background extinctions because they have occurred regularly since life originated on Earth (2–4). The fossil record of biota also reveals mass extinctions as distinct phenomena that resulted in a succession of family assemblages that dominated for a time and then were replaced. Mass extinctions may be identified at first by a drop in biodiversity that reflects exceptional ecologic crisis, usually considered as devastating the biosphere. For example, at the end of the Permian era, some 250 million years ago, approximately 80% of species became extinct. Based on a detailed census of the end of the Permian event and the C-T mass extinction, everything from microscopic foraminifera to tetrapods seems to have vanished in an extremely short period (3,5). The C-T episode is not the only major mass extinction phenomenon during the last 540-million–year period for which abundant fossil remains were found, but the fact that it led to the transition from the age of the dinosaurs to the age of mammals about 65 million years ago puzzled both scientists and the public for more than a century. Although more than 60 theories have been developed to explain this mass extinction event, the selectivity of the mass extinction for dinosaurs and their contemporary vertebrates and invertebrates remains unexplained. For example, among the 12 major vertebrate clades, extinctions were concentrated in seven of them; sharks, ornithischians, pterosaurus, saurischians, non-trilasghemic mammals, lizards, and marsupials accounted for about 75% of the extinctions (4). This demonstrates that the C-T extinctions were highly selective. Moreover, survivorship at least for the lizards and marsupials varied significantly in different geographic regions. The same pattern of concentrations in certain families is also the main feature in the paleocene floral diversities and turnover events at that time (6). Therefore, any theory of extinction must account for this selectivity as well as for the diachroneity found in the biological record during the late Cretaceous/early Tertiary period. My hypothesis for the cause of mass extinction during the C-T period is that selective toxic effects from an exceptionally high environmental pollution occurred at that time. Iridium, a well-known tracer of the C-T boundary during the turnover of biodiversity, can be used to check the hypothesis that global chemical stress is a significant cause of the loss of biodiversity and selectivity during C-T mass extinction. The iridium anomaly seems to be related to a large quantity of siderolite material most

Paleoecotoxicology: the impact of chemical and physical stress in the evolutionary process.