You as a leaf on the Tree of Life
Because living organisms on planet earth all evolved from a common cellular ancestor, each living organism is a leaf on the enormous tree of life. Branches in the tree of life represent the connections among all species on the planet. Individuals are connected through a network of branches within species, species are connected through branches representing their history of diversification, and major groups of organisms are connected through successive branches back to the original diversification from the single-celled ancestor. Richard Dawkins in his 2009 book The Greatest Show on Earth provides an excellent metaphor for the connections or nodes in the Tree of Life,
“...for any two animals there has to be a hairpin path linking them, for the simple reason that every species shares an ancestor with every other species: all we have to do is walk backwards from one species to the shared ancestor, then turn through a hairpin bend and walk forwards to the other species.”
The same applies for people, considering that some individuals share with each other a more recent common ancestor(s) than with others. The webpage Genetic Connections Between Organisms from the Tree of Life Project nicely relates the genealogical connections among individuals within a family to the branching pattern within the tree of life. Because DNA contains the hereditary information for all cellular life, the branching history within the tree of life is recorded in the diversity of DNA sequences present among living organisms. Personal genomics enables an individual to discover their place within the tree of life.
Biologists represent relationshipsamong organisms using phylogenetic trees. The tips, or leaves, of a phylogenetic tree can represent a variety of biological forms, such as DNA sequences, individual organisms, or species and other taxonomic groupings of organisms. In the image to the right, a hypothetical set of organismal forms is represented at the tips of a phylogenetic tree. Branches in the tree reflect lines of descent, which are connected at nodes that represent common ancestry of descendant forms. Subsets of more closely related organisms are connected at nodes reflecting unique common ancestors of that group. For example organisms C and D are connected through a unique common ancestor, thus C and D are more closely related to each other than to A, B and E. At the very base of the tree on the left-hand side, the single common ancestor is represented for all organisms at the tips of the tree. Chronological time in this diagram of a phylogenetic tree moves from left to right, with the most ancient split between lineages on the left and the representation of living organisms on the right.
Pedigrees and family trees are specialized types of phylogenetic trees commonly used to illustrate relationships in human families. Since humans reproduce sexually, each node within a pedigree consists of a pair of ancestors through which descendant lineages are produced. For example, your siblings are connected to you through your parents, your 1st cousins are connected to you through your grandparents, your 2nd cousins through your great grandparents, etc.
Relationships within a phylogenetic tree represent a hierarchical ordering of common ancestry. Each node connects lines of descent from a common ancestor. This series of historical relationships can be reconstructed from the characteristics of organisms. Presence of shared features indicates common ancestry, because evolutionary changes that accumulate in a lineage are passed along to descendant lineages. For example, the presence of a mid-section in two of the representative hypothetical organisms indicates that they share a common ancestor which evolved this feature. Likewise, the other grouping reflects descent from a common ancestor that evolved legs. Note that in each of these cases the evolutionary changes have undergone modifications in descendant lineages. In essence the characteristics of living organisms provide a record of evolutionary history, and a variety of methods have been developed for deciphering phylogenetic relationships by comparison of characteristics among living organisms. Similar methods are used in the analysis of personal genetic data to estimate relationships among humans.
Since the DNA present in each cell provides the recipe for cellular life, and this recipe is copied and transmitted as cells divide and organisms reproduce, DNA is an excellent source of characteristics for reconstructing historical relationships. The sequence of bases in DNA molecules change through the process of mutation. Mutation is rare, but persistent, so that DNA sequences slowly accumulate changes over generations. Shared changes in DNA sequence provide evidence for reconstructing patterns of common ancestry. Ultimately, however, phylogenetic reconstruction is an inference (or hypothesis) describing biological relationships.
Genetic Genealogy: The Intersection of Phylogenetics and Genealogy
Application of DNA analysis to investigate ancestry enables an individual to support the historical record and to discover relationships that are misrepresented in the historical record or otherwise not documented. Genetic tools simply analyze shared features in DNA sequence data to infer recent common ancestry. In a database of individual genetic data, some individuals are more similar to each other due to the inheritance of DNA from a more recent common ancestor(s). Within a phylogenetic context, these related individuals are connected by a node reflecting this recent common ancestry. Estimates of whether this common ancestry is through great great grandparents, a patrilineal grandfather 10 generations removed, or a common maternal line that originated 4,000 years ago can be made and are dependent on the type of genetic data analyzed. In the ideal genetic genealogy world, all ancestral connections among all living individuals would be inferred through a network of branches connected through nodes representing all common ancestors. However, current tools are unable to provide this level of resolution beyond a few generations, so we may never be able to reveal our genetic ancestry at this level of detail. In a genealogical context, genetic data are used only to infer common ancestry, most often based on analysis of genetic data from related individuals that each descend from a set of common ancestors, some more recent than others, and not to identify these ancestors.
Genealogical research based on historical records provides the human tapestry of this ancestry with names, events, dates and places. Use of genetic tools to reconstruct common ancestry may be complementary, thus supportive, of historical records. It may also be contradictory when the history does not accurately reflect the biology. Recognize, however, that genetic data does not validate or confirm or prove the historical record except in direct comparisons between close relatives when no other relationships are plausible given the genetic data (i.e., parent-child, full siblings). Alternative historical scenarios that are not represented, or are misrepresented, in the historical record are often equally consistent with genetic data. The intersection between genetics and the historical record in the context of genealogical research is quite analogous to the intersection between molecular systematics and paleontology. Molecular systematists aim to define the relationships among species in the tree of life and paleontologists aim to describe and place fossil forms relative to common ancestral nodes. Just as the fossil record sparsely represents the biodiversity that has existed on the planet, the historical record can bias and in cases where it is missing leave much of one's genetic ancestry lacking in personality.