Biological explanations

Biological explanations

Early naturalists well understood the similarities and differences of living species leading Linnaeus to develop a hierarchical classification system still in use today. It was Darwin and his contemporaries who first linked the hierarchical structure of the great tree of life in living organisms with the then very sparse fossil record. Darwin eloquently described a process of descent with modification, or evolution, whereby organisms either adapt to natural and changing environmental pressures, or they perish.

Petrified cone of Araucaria sp. from Patagonia,Argentina dating from the Jurassic Period (approx. 210 Ma)

When Charles Darwin wrote On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, the oldest animal fossils were those from the Cambrian Period, now known to be about 540 million years old. The absence of older fossils worried Darwin about the implications for the validity of his theories, but he expressed hope that such fossils would be found, noting that: "only a small portion of the world is known with accuracy." Darwin also pondered the sudden appearance of many groups (i.e. phyla) in the oldest known Cambrian fossiliferous strata.

Further discoveries

Since Darwin's time, the fossil record has been pushed back to between 2.3 and 3.5 billion years before the present.Most of these Precambrianfossils are microscopic bacteria or microfossils. However, macroscopic fossils are now known from the late Proterozoic. The Ediacara biota (also calledVendian biota) dating from 575 million years ago collectively constitutes a richly diverse assembly of early multicellular eukaryotes.

The fossil record and faunal succession form the basis of the science of biostratigraphy or determining the age of rocks based on the fossils they contain. For the first 150 years of geology, biostratigraphy and superposition were the only means for determining the relative age of rocks. The geologic time scale was developed based on the relative ages of rock strata as determined by the early paleontologists and stratigraphers.

Since the early years of the twentieth century, absolute dating methods, such as radiometric dating (including potassium/argon, argon/argon, uranium series, and, for very recent fossils, radiocarbon dating) have been used to verify the relative ages obtained by fossils and to provide absolute ages for many fossils. Radiometric dating has shown that the earliest known stromatolites are over 3.4 billion years old. Various dating methods have been used and are used today depending on local geology and context, and while there is some variance in the results from these dating methods, nearly all of them provide evidence for a very old Earth, approximately 4.6 billion years.

Modern view

"The fossil record is life’s evolutionary epic that unfolded over four billion years as environmental conditions and genetic potential interacted in accordance with natural selection.The earth’s climate, tectonics, atmosphere, oceans, and periodic disasters invoked the primary selective pressures on all organisms, which they either adapted to, or they perished with or without leaving descendants. Modern paleontology has joined with evolutionary biology to share the interdisciplinary task of unfolding the tree of life, which inevitably leads backwards in time to the microscopic life of thePrecambrian when cell structure and functions evolved. Earth’s deep time in the Proterozoic and deeper still in the Archean is only "recounted by microscopic fossils and subtle chemical signals^]Molecular biologists, using phylogenetics, can compare protein amino acid or nucleotide sequence homology (i.e., similarity) to infer taxonomy and evolutionary distances among organisms, but with limited statistical confidence. The study of fossils, on the other hand, can more specifically pinpoint when and in what organism branching occurred in the tree of life. Modern phylogenetics andpaleontology work together in the clarification of science’s still dim view of the appearance of life and its evolution during deep time on earth.

Phacopid trilobite Eldredgeops rana crassituberculata, the genus is named after Niles Eldredge

Crinoid columnals (Isocrinus nicoleti) from the Middle Jurassic Carmel Formation atMount Carmel Junction, Utah; scale in mm

Niles Eldredge’s study of the Phacops trilobite genus supported the hypothesis that modifications to the arrangement of the trilobite’s eye lenses proceeded by fits and starts over millions of years during theDevonian.Eldredge's interpretation of the Phacops fossil record was that the aftermaths of the lens changes, but not the rapidly occurring evolutionary process, were fossilized. This and other data led Stephen Jay Gould and Niles Eldredge to publish the seminal paper on punctuated equilibrium in 1971.

Example of modern development

An example of modern paleontological progress is the application of synchrotron X-ray tomographictechniques to early Cambrian bilaterian embryonic microfossils that has recently yielded new insights ofmetazoan evolution at its earliest stages. The tomography technique provides previously unattainable three-dimensional resolution at the limits of fossilization. Fossils of two enigmatic bilaterians, the worm-likeMarkuelia and a putative, primitive protostome, Pseudooides, provide a peek at germ layer embryonic development. These 543-million-year-old embryos support the emergence of some aspects of arthropoddevelopment earlier than previously thought in the late Proterozoic. The preserved embryos from China andSiberia underwent rapid diagenetic phosphatization resulting in exquisite preservation, including cell structures. This research is a notable example of how knowledge encoded by the fossil record continues to contribute otherwise unattainable information on the emergence and development of life on Earth. For example, the research suggests Markuelia has closest affinity to priapulid worms, and is adjacent to the evolutionary branching of Priapulida, Nematoda and Arthropoda.

Developments in interpretation of the fossil record

Developments in interpretation of the fossil record

Silurian Orthoceras Fossil

Ever since recorded history began, and probably before, people have noticed and gathered fossils, including pieces of rock and minerals that have replaced the remains of biologic organisms, or preserved their external form. Fossils themselves, and the totality of their occurrence within the sequence of Earth's rock strata, is referred to as the fossil record.

The fossil record was one of the early sources of data relevant to the study of evolution and continues to be relevant to the history of life on Earth. Paleontologistsexamine the fossil record in order to understand the process of evolution and the way particular species have evolved.

Explanations

Fossil shrimp (Cretaceous)

A fossil gastropod from the Plioceneof Cyprus. A serpulid worm is attached.

Various explanations have been put forth throughout history to explain what fossils are and how they came to be where they were found. Many of these explanations relied on folktales or mythologies. In China the fossil bones of ancient mammals including Homo erectus were often mistaken for “dragon bones” and used as medicine and aphrodisiacs. In the West the presence of fossilized sea creatures high up on mountainsides was seen as proof of the biblical deluge.

Greek scholar Aristotle realized that fossil seashells from rocks were similar to those found on the beach, indicating the fossils were once living animals.Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life.In 1027, the Persian geologist, Avicenna explained how thestoniness of fossils was caused in The Book of Healing. However, he rejected the explanation of fossils as organic remains.Aristotle previously explained it in terms of vaporous exhalations, which Avicenna modified into the theory of petrifying fluids (succus lapidificatus), which was elaborated on by Albert of Saxony in the 14th century and accepted in some form by most naturalists by the 16th century. Avicenna gave the following explanation for the origin of fossils from thepetrifaction of plants and animals:

If what is said concerning the petrifaction of animals and plants is true, the cause of this (phenomenon) is a powerful mineralizing and petrifying virtue which arises in certain stony spots, or emanates suddenly from the earth during earthquake and subsidences, and petrifies whatever comes into contact with it. As a matter of fact, the petrifaction of the bodies of plants and animals is not more extraordinary than the transformation of waters.

More scientific views of fossils emerged during the Renaissance. For example, Leonardo Da Vinci noticed discrepancies with the use of the biblical flood narrative as an explanation for fossil origins:

"If the Deluge had carried the shells for distances of three and four hundred miles from the sea it would have carried them mixed with various other natural objects all heaped up together; but even at such distances from the sea we see the oysters all together and also the shellfish and the cuttlefish and all the other shells which congregate together, found all together dead; and the solitary shells are found apart from one another as we see them every day on the sea-shores.

And we find oysters together in very large families, among which some may be seen with their shells still joined together, indicating that they were left there by the sea and that they were still living when the strait of Gibraltar was cut through. In the mountains of Parma and Piacenza multitudes of shells and corals with holes may be seen still sticking to the rocks...

William Smith (1769–1839), an English canal engineer, observed that rocks of different ages (based on the law of superposition) preserved different assemblages of fossils, and that these assemblages succeeded one another in a regular and determinable order. He observed that rocks from distant locations could be correlated based on the fossils they contained. He termed this the principle of faunal succession.

Smith, who preceded Charles Darwin, was unaware of biological evolution and did not know why faunal succession occurred. Biological evolution explains why faunal succession exists: as different organisms evolve, change and go extinct, they leave behind fossils. Faunal succession was one of the chief pieces of evidence cited by Darwin that biological evolution had occurred.

Georges Cuvier came to believe that most if not all the animal fossils he examined were remains of species that were now extinct. This led Cuvier to become an active proponent of the geological school of thought called catastrophism. Near the end of his 1796 paper on living and fossil elephants he said:

All of these facts, consistent among themselves, and not opposed by any report, seem to me to prove the existence of a world previous to ours, destroyed by some kind of catastrophe.