8.2: Evolution and the Tree of Life

One and one half centuries after Darwin's work, modern genetic science has unequivocally confirmed that all life is related. The Tree of Life is also clearly encoded in the fossil record, even if there exist gaps in the stone sequences. At a macroscopic level, modern theory of evolution is based on two primary tenets:

• All living things are related to one another to varying degrees through common descent (share common ancestors), have developed from other species, and all life forms have a common ancestor.
• The origin of a new species results from random heritable genetic mutations (changes), some of which are more likely to spread and persist in a gene pool than others. Mutations that result in an advantage to survive and reproduce are more likely to be retained and propagated than mutations that do not result in a survival to reproductive advantage.

Descent with modification, or evolution, is often described by the metaphorical Tree of Life. A tree is inherently hierarchical, as is the great "Tree of Life". Its boughs are analogous to the higher Linnaean rankings, i.e., the domains, kingdoms, phyla, classes, etc. Smaller branches correspond to middle rankings, i.e., the orders, families and genera. At the end of the many branches are the twigs, the uncountable species, some 99% of which are extinct.

Figure \(\PageIndex{1}\): Dimetrodon

Mammals evolved from synapsids (also called mammal-like reptiles, but are better referred to as stem-mammals, and are not reptiles). Dimetrodon is a synapsid example from the Permian. Mammal evolution was a gradual, extended process that spanned some approximately 70 million years, from about the middle Permian to the Middle Jurassic. By the middle Triassic, many animals had appeared that looked like mammals. Hadrocodium wui was a basal mammal species that lived during the early Jurassic; it was discovered in the famous Lufeng Basin in Yunnan Province, southwestern China. While Hadrocodium did not have all mammal characteristics, it did have a separate jawbone, large brain, and sophisticated hearing. It weighed a miniscule two grams. Whether Hadrocodium was warm- or cold-blooded remains in dispute. It co-existed with several other primitive mammals with much larger body size.

Dimetrodon, meaning "two-measures tooth," was a reptile that lived in the Permian Period, living between 280 and 265 million years ago. It is believed to be more closely related to mammals than to reptiles (Sauropsida) such as dinosaurs, lizards and birds. Dimetrodon fossils have been found in North America and Europe.

Figure \(\PageIndex{2}\): Dimetrodon skeleton on display in a museum

Growing up to 10 feet in length, and possessing a large head with large canine-like teeth, it was a top carnivore during part of Permian time. Dimetrodon had a large sail on its back that was probably used to regulate body temperature much like the radiator in a car. The sail may have also provided camouflage when it lurked in bamboo-like Calamite plants.

Cambrian Explosion

Most major animal groups appear for the first time in the fossil record some 545 million years ago on the geological time scale in a relatively short period of time known as the Cambrian explosion. Of great worry to Darwin, the explanation of this sudden, apparent explosion persists as a source of numerous major debates in paleobiology. While some scientists believe there was indeed an explosion of diversity (the so-called Punctuated Equilibrium theory elaborated by Nils Eldredge and the late Stephen J. Gould - Models In Paleobiology, 1972), others believe that such rapid acceleration of evolution is not possible; they posit that there was an extended period of evolutionary progression of all the animal groups, but the evidence for this was lost in the Precambrian fossil record. Early complex animals in the Paleozoic may have been nearly microscopic. Fossil animals smaller than 0.2 mm have been found in the Doushantuo Formation, China, 40-55 million years before the Cambrian period (Chen et al. 2004). Much of their early evolution could have simply been too small to see. Modern molecular technologies (genomics, for example), through comparing nucleic acid and amino acid sequences across living species, are enabling scientists to identify genetic components and patterns conserved by evolution; from these the evolutionary branching of the Tree of Life can be inferred.

Figure \(\PageIndex{3}\): The effect of the Cambrian explosion

The theory of the Cambrian Explosion holds that, beginning some 545 million years ago, an explosion of diversity led to the appearance over a relatively short period of 5 million to 10 million years of a huge number of complex, multi-celled organisms. Moreover, this burst of animal forms led to most of the major animal groups we know today, that is, every extant Phylum. It is also postulated that many forms that would rightfully deserve the rank of and Phylum both appeared in the Cambrian only to rapidly disappear. Natural selection is generally believed to have favored larger size, and consequently the need for hard skeletons to provide structural support - hence, the Cambrian gave rise to the first shelled animals and animals with exoskeletons (e.g., the trilobites). With the innovation of structural support, the early Cambrian period also saw the start of an explosion in the size of many animals.

Figure \(\PageIndex{4}\): Cambrian Animals

The Cambrian Explosion is the outcome of changes in environmental factors leading to changes in selective pressures, in turn leading to adaptive diversification on a vast scale. By the start of the Cambrian, the large supercontinent Gondwana, comprising all land on Earth, was breaking up into smaller land masses. This increased the area of continental shelfs produced shallow seas, thereby also expanding the diversity of environmental niches in which animals could specialize and speciate.

The debate persists today about whether the evolutionary "explosion" of the Cambrian was as sudden and spontaneous as it appears in the fossil record. The discovery of new pre-Cambrian and Cambrian fossils help resolve the debate, as these transitional fossil forms support the hypothesis that diversification was well underway before the Cambrian began. More recently, the sequencing of the genomes of thousands of life forms is revealing just how many and what genes and the proteins they encode have been conserved from the Precambrian.

It is important to remember that geological history contains numerous periods of slow evolution punctuated by periods of rapid evolution, which Steven J. Gould called Punctuated Equilibrium. The rates of evolution generally depend on rates of selection, which in turn depend on rates of environmental change. It also depends upon the existing genomic diversity on which selection acts. Mutation rates tend to be slow and steady, and in the absence of environmental change, slowly accumulate in a population. It is selective pressure that weeds out the mutations that are detrimental or neutral to survival, and retains and amplifies the mutations that are beneficial within a population. For a population isolated in a new environment, rapid selection can lead to speciation, and in the Lower Cambrian, to radically new forms that we now group in the Phyla of modern times occurred to an unprecedented extent that has never since been repeated.

Transitional Fossils

What Are Transitional Fossils

Transitional fossils are the fossilized remains of transitional forms of life that tangibly and demonstrably encode an evolutionary transition. Thus, transitional fossils are characterized by their retention of primitive (plesiomorphic) traits in contrast with their more recently evolved characteristics (the phenotype and genotype).

The term "missing link" is a popular slang term for such transitional forms, but is misleading. The term is particularly used in popular media, but is inaccurate and confusing, partly because it implies that there exists a single undiscovered fossil that is needed to confirm the transition. In contrast, the continual discovery of more and more transitional fossils is further refining and validating evolutionary transitions. Transitional fossils are numerous and varied throughout the tree of life, including those between primates and early humans, contrary to the claims of creationists who deny evolution.

Evolutionary theory considers all populations of organisms to be in transition, whether changes be slow, as in genetic drift, or fast, as when a changing environment imposes significant adaptive pressures. A transitional form of life is one that demonstrably illustrates a particular intermediate evolutionary stage of change or adaptation.

Transitional fossils usually coexist with gaps in a sequence in the fossil record. The probabilities of fossilization pretty much preclude the discovery of detailed sequences of fossils spanning millions of years. However, fine gradations of fossils between species and genera are abundant in the fossil record, as are coarser sequences between higher taxa.

Examples: Transition from fish to amphibian: Tiktaalik— a fish with developing legs. Also appearance of ribs and neck.

Human ancestors (transition to bipedal walking): Sahelanthropus tchadensis— One of the oldest known species in the human family tree. Lived around 6.5-7 million years

Contributors and Attributions

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