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

day biodiversity, estimated to comprise more than 100 million 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 pre-human 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 …