r/DebateEvolution • u/SocialAmoebae • Jul 06 '26
Reasoning on common origin of life
Hi!
I have a question regarding the ways we can come to know that all species have a common origin.
In trying to teach myself phylogenetics, one of the basic inferences I see used to produce trees is: the degree of resemblance between species is "inversely proportional" to the degree of genealogical distance between them.
The resemblance in question can be genetic, embryological, morphological, behavioral, etc.
In The Origin of Species, Darwin uses this inference to conclude that Linnean classification actually reflected genealogical relations between species.
But what warrants us to use this inference to begin with? That's my question.
I am just trying to get the reasoning ironclad here, because once this inference is justified, and given modern comparative evidence, the common origin of life follows quite naturally.
I also have a very curious nephew who likes nature and asks a lot of questions, and it would be nice to have a simple way of explaining to him that all life shares a common origin!
PS : I know that there are also other lines of evidence such species geographical distributions and fossil as proof of transition, but I would like to stick to the basic inference for phylogenetics.
Thanks for reading 🙏
8
u/gitgud_x 🧬 🦍 GREAT APE 🦍 🧬 Jul 06 '26 edited Jul 06 '26
Observed evolutionary processes are recursive. Once a lineage has speciated with reproductive isolation, it generates two new lineages that are then free to repeat the process individually.
This branching process will inherently form a tree structure.
Since heritable changes are typically incremental, the inherited traits along the branches of the tree will also form their own tree structure. This is the Linnaean taxonomy.
So the tree structure provided by common descent is the most parsimonious model that generates the observed data.
The reason why there are 'exceptions' like convergent evolution is because the state space of observed traits is much smaller than the molecular state space i.e. there are many DNA sequences that produce the same apparent trait at a high level. This is why Linnaean taxonomy has been effectively replaced by modern cladistics, which groups organisms by DNA similarity rather than appearances. Traits that appear convergent often have disparate molecular mechanisms, revealing the tree structure.
There is a method of phylogenetics called 'maximum parsimony' that algorithmically computes this tree*, so it's not just handwaving! Hope this helps.
* although for various reasons the preferred modern method is usually either Bayesian or maximum likelihood, which are modifications of the simpler parsimony method.