Is there a tree of life?

Tree of life

Many biologists would not hesitate to answer this question in the affirmative. However, in these lines I am going to raise the possibility that the answer should be negative. Let us see why.

First of all, what is a tree? When we use this term, talking about the tree of life, we don’t mean a biological tree, i.e. a plant with a woody trunk that grows to a height of several (or many) meters. We mean a mathematical tree. But what is that?

A branch of mathematics called graph theory deals with sets of objects and represents graphically the relationships between those objects. A graph is made up of circles or nodes, the objects in the set, and arrows or arcs, which are lines connecting the objects when there is a relationship between them. When this relationship has the reciprocal property (i.e. if A is related to B, then B is related to A) the arcs will be bidirectional and we’ll represent them by a two-headed arrow; if this property doesn’t hold, then the arrow’s head will be just at one end, for the existence of a relationship between A and B does not imply that there is a relation between B and A. Let’s look at two special types of graphs:

• 

  Connected graph
  

  A graph is said to be connected if every node is joined to all the others by a bidirectional arc.
• A tree is a graph with the following four properties: a) The relation represented by the graph is never bidirectional. b) The graph has no loops (closed circuits). c) Every node except one receive a single incoming arc, although they can have any number of outgoing arcs. d) There is a distinguished node (usually called the root), not reached by any arc, located at the start of paths (sequences of arcs) that go through all the other nodes in the tree.

It is traditional to represent evolution as a tree where the objects are biological species and the relationship is descent, i.e. if one species descends from another we draw an arc from the parent species node to the descendant species node. In this tree there is a supreme node or root, at the top of the tree, usually called LUCA, from the initials of Last Universal Common Ancestor. The node labeled LUCA is assumed to represent the origin of life.

In this post I’m questioning the existence of the tree of life. To prove it, I’m offering the following reasons:

1. Some species do not descend from a single ancestor species, but from several. For instance, the first eukaryote arose from the symbiosis of an anaerobic archaea with aerobic bacteria that kept living inside the other cell and became its mitochondria. Later, some eukaryotes performed a new symbiosis, this time with cyanobacteria capable of photosynthesis, which became their chloroplasts. Lichens also descend from more than one ancestor, having arisen from the symbiosis of algae and fungi. Finally, practically all species, including ours, have pieces of genomes of viruses and other microorganisms in the genome, which means that they haven’t inherited their genomes from a single ancestor species, but from several.
2. Some groups of living beings, such as bacteria and archaea, can exchange genetic information through plasmids: DNA chains that pass from one microorganism to another, which can be of a different species, and are incorporated into the genome of the second. That is why it is often said that bacteria and archaea don’t really form a set of species, but a genetic continuum, a field of life without obvious divisions into different species. See this post in my blog. Therefore, the parts of the tree of life corresponding to bacteria and archaea should be represented, not as a tree, but as a connected graph.
3. Finally, faced with this situation, I think it’s appropriate to consider the possibility that LUCA (our oldest ancestor) never existed, but that it must also be replaced by a connected graph, from which in turn would have arisen the bacteria and the archaea.

The following figure represents how I think should be represented the graph that would replace the tree of life, taking into account the three previous considerations.

Thematic Thread on Primitive Life: Previous Next

The same post in Spanish

Manuel Alfonseca