Charles DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
believed that phylogeny, the ascent of all species through time, was expressible as a metaphor he termed the
Tree of Life. The modern development of this idea is called the
Phylogenetic treeA phylogenetic tree or evolutionary tree is a tree showing the evolutionary relationships among various biological species or other entities that are believed to have a common ancestor. In a phylogenetic tree, each node with descendants represents the most recent common ancestor of the...
.
Jean-Baptiste LamarckJean-Baptiste Pierre Antoine de Monet, Chevalier de la Marck , often just known as 'Lamarck', was a French soldier, naturalist, academic and an early proponent of the idea that evolution occurred and proceeded in accordance with natural laws.Lamarck fought in the Pomeranian War with Prussia, and...
(1744-1829) produced a simple branched tree of animals in his
Philosophy zoologique (1809). It was an inverted tree starting with worms and ending with mammals. However, Lamarck did not believe in common descent of all life.
Charles DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
believed that phylogeny, the ascent of all species through time, was expressible as a metaphor he termed the
Tree of Life. The modern development of this idea is called the
Phylogenetic treeA phylogenetic tree or evolutionary tree is a tree showing the evolutionary relationships among various biological species or other entities that are believed to have a common ancestor. In a phylogenetic tree, each node with descendants represents the most recent common ancestor of the...
.
Early Trees of Life
Jean-Baptiste LamarckJean-Baptiste Pierre Antoine de Monet, Chevalier de la Marck , often just known as 'Lamarck', was a French soldier, naturalist, academic and an early proponent of the idea that evolution occurred and proceeded in accordance with natural laws.Lamarck fought in the Pomeranian War with Prussia, and...
(1744-1829) produced a simple branched tree of animals in his
Philosophy zoologique (1809). It was an inverted tree starting with worms and ending with mammals. However, Lamarck did not believe in common descent of all life. In stead, he believed that life consists of separate parallel lines advancing from simple to complex. The American geologist
Edward HitchcockEdward Hitchcock was a noted American geologist and the third President of Amherst College .Born to poor parents, he attended newly-founded Deerfield Academy and in 1821 was ordained as a Congregationalist pastor. A few years later he left the ministry to become Professor of Chemistry and Natural...
(1763–1864) published in 1840 the first Tree of Life based on paleontology in his
Elementary Geology. Although it is a branching tree, one tree for plants and one for animals, it is not an evolutionary tree, because Hitchcock believed that a deity was the agent of change.
Darwin's Tree of Life
Charles Darwin (1809-1882) was the first to produce an evolutionary tree of life. He was very cautious about the possibility of reconstructing the history of life. In
On the Origin of Species (1859) he presented a theoretical Tree of Life with no real species names (see figure). On the horizontal axis are hypothetical species labeled A - L and on the vertical axis are historical periods labeled I - XIV. In the figure the lines A - L do not originate from one point of origin, although they seem to converge to one point. The purpose of the figure is to demonstrate branching (A,I), extinction (B,C,D,E,G,H,K,L) and constancy (F). In Darwin's own words: "Thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species". Compare Darwin's Tree of Life with the one Ernst Haeckel made years later (figure below) which includes the names of species. An excerpt from Darwin's
On the Origin of Species explaining his views on the
Tree of Life:
Haeckel's Tree of Life
Ernst HaeckelErnst Heinrich Philipp August Haeckel ,also written von Haeckel, was an eminent German biologist, naturalist, philosopher, physician, professor and artist who discovered, described and named thousands of new species, mapped a genealogical tree relating all life forms, and coined many terms in...
(1834 - 1919) constructed several Trees of Life. Left is shown the first sketch of the famous Haeckel's Tree of Life in the 1860s which shows "Pithecanthropus alalus" as the ancestor of Homo sapiens. In the middle is the Tree of Life from
Generelle Morphologie der Organismen (1866) with three kingdoms Plantae, Protista and Animalia. On the right is the 'Pedigree of Man' published in
The Evolution of Man (1879).
The tree of life today
The model is still considered valid for eukaryotic life forms. The earliest branch of the eukaryote tree yields four supergroups: Plants (green and red algae, and plants),
UnikontUnikonts are members of the Unikonta, a taxonomic group proposed by Thomas Cavalier-Smith. It includes amoebozoa and opisthokonts.-Clade:The group includes eukaryotic cells that, for the most part, have a single emergent flagellum, or are amoebae with no flagella. The unikonts include opisthokonts...
s (amoebas, fungi, and all animals—including humans),
ExcavateThe excavates are a major assemblage of unicellular eukaryotes, often known as Excavata. The phylogenetic category Excavata contains a variety of free-living and symbiotic forms, and includes some important parasites of humans.-Characteristics:...
s (free-living organisms and parasites), and SAR (a recently identified main group, abbreviated from Stramenopiles,
AlveolateThe alveolates are a major line of protists.-Phyla:There are three phyla, which are very divergent in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:...
s, and
RhizariaThe Rhizaria are a species-rich supergroup of unicellulareukaryotes. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. Many produce shells or skeletons, which may be quite complex in structure, and these make up...
, the names of some of its members).
Biologists now recognize, that the prokaryotes, the
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
, have the ability to transfer genetic information between unrelated organisms through
Horizontal gene transferHorizontal gene transfer , also Lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g...
(HGT). Recombination, gene loss, duplication, and gene creation are a few of the processes by which genes can be transferred within and between bacterial and archael species, causing variation that’s not due to vertical transfer.
There is emerging evidence of HGT occurring within the prokaryotes at the single and multicell level and the view is now emerging that the tree of life gives an incomplete picture of life's evolution. It was a useful tool in understanding the basic processes of evolution but cannot explain the full complexity of the situation.
See also
- Cladistics
Cladistics is a form of biological systematics which classifies living organisms on the basis of shared ancestry...
- Common descent
A group of organisms is said to have common descent if they have a common ancestor. In modern biology, it is generally accepted that all living organisms on Earth are descended from a common ancestor or ancestral gene pool....
- Phylogenetic tree
A phylogenetic tree or evolutionary tree is a tree showing the evolutionary relationships among various biological species or other entities that are believed to have a common ancestor. In a phylogenetic tree, each node with descendants represents the most recent common ancestor of the...
- Tree of Life Web Project
The Tree of Life Web Project is an ongoing Internet project providing information about the diversity and phylogeny of life on Earth. This collaborative peer reviewed project began in 1995, and is written by biologists from around the world....
- Horizontal gene transfer
Horizontal gene transfer , also Lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g...
External links