Transitional fossil
Darwin described the perceived lack of transitional fossils as "the most obvious and gravest objection which can be urged against my theory," but he explained it by relating it to the extreme imperfection of the geological record.
[3] He noted the limited collections available at the time but described the available information as showing patterns that followed from his theory of descent with modification through natural selection.
The term "missing link" has been used extensively in popular writings on human evolution to refer to a perceived gap in the hominid evolutionary record.
[7] With the establishment of cladistics in the 1990s, relationships commonly came to be expressed in cladograms that illustrate the branching of the evolutionary lineages in stick-like figures.
[12] It lived in what is now southern Germany in the Late Jurassic period around 150 million years ago, when Europe was an archipelago in a shallow warm tropical sea, much closer to the equator than it is now.
Similar in shape to a European magpie, with the largest individuals possibly attaining the size of a raven,[13] Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length.
[17]: 122 While the pelvis is not entirely like that of a human (being markedly wide, or flared, with laterally orientated iliac blades), these features point to a structure radically remodelled to accommodate a significant degree of bipedalism.
[22][page needed] Pakicetids could hear under water, using enhanced bone conduction, rather than depending on tympanic membranes like most land mammals.
[26] Their diet probably included land animals that approached water for drinking, or freshwater aquatic organisms that lived in the river.
Tiktaalik is a genus of extinct sarcopterygian (lobe-finned fish) from the Late Devonian period, with many features akin to those of tetrapods (four-legged animals).
Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod.
"[30][31] Unlike many previous, more fish-like transitional fossils, the "fins" of Tiktaalik have basic wrist bones and simple rays reminiscent of fingers.
Like all modern tetrapods, it had rib bones, a mobile neck with a separate pectoral girdle, and lungs, though it had the gills, scales, and fins of a fish.
[32] Tetrapod footprints found in Poland and reported in Nature in January 2010 were "securely dated" at 10 million years older than the oldest known elpistostegids[33] (of which Tiktaalik is an example), implying that animals like Tiktaalik, possessing features that evolved around 400 million years ago, were "late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates.
Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the seabed (benthos), and the extension of the dorsal fin onto the head.
The dramatic find of the London specimen of Archaeopteryx in 1861, only two years after the publication of Darwin's work, offered for the first time a link between the class of the highly derived birds, and that of the more basal reptiles.
[43] In a letter to Darwin, the palaeontologist Hugh Falconer wrote: Had the Solnhofen quarries been commissioned—by august command—to turn out a strange being à la Darwin—it could not have executed the behest more handsomely—than in the Archaeopteryx.
[45] For example, the Swedish encyclopedic dictionary Nordisk familjebok of 1904 showed an inaccurate Archaeopteryx reconstruction (see illustration) of the fossil, "ett af de betydelsefullaste paleontologiska fynd, som någonsin gjorts" ("one of the most significant paleontological discoveries ever made").
In 1917, Robert Kidston and William Henry Lang found the remains of an extremely primitive plant in the Rhynie chert in Aberdeenshire, Scotland, and named it Rhynia.
The unusual mix of moss-like and vascular traits and the extreme structural simplicity of the plant had huge implications for botanical understanding.
[51] After On the Origin of Species, the idea of "lower animals" representing earlier stages in evolution lingered, as demonstrated in Ernst Haeckel's figure of the human pedigree.
[53] Lyell's vivid writing fired the public imagination, inspiring Jules Verne's Journey to the Center of the Earth (1864) and Louis Figuier's 1867 second edition of La Terre avant le déluge ("Earth before the Flood"), which included dramatic illustrations of savage men and women wearing animal skins and wielding stone axes, in place of the Garden of Eden shown in the 1863 edition.
[54] The search for a fossil showing transitional traits between apes and humans, however, was fruitless until the young Dutch geologist Eugène Dubois found a skullcap, a molar and a femur on the banks of Solo River, Java in 1891.
The find combined a low, ape-like skull roof with a brain estimated at around 1000 cc, midway between that of a chimpanzee and an adult human.
The single molar was larger than any modern human tooth, but the femur was long and straight, with a knee angle showing that "Java Man" had walked upright.
At the time it was hailed by many as the "missing link," helping set the term as primarily used for human fossils, though it is sometimes used for other intermediates, like the dinosaur-bird intermediary Archaeopteryx.
[5][60] The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge and first presented in 1972[61] is often mistakenly drawn into the discussion of transitional fossils.
