Hi Carlos,

I would use the adjective "ancestral" for organisms that first appeared in the history of life.

"Ancestral" or "basal" would be appropriate in a phylogenetic context. Evolution does not move up a ladder of complexity, so terminology should avoid that implication. Loss of complexity can be an evolutionary advantage, and thus a direction some organisms will follow.

I find "primitive" to remain a valuable term that helps us understand traits closer to phylogenetic "roots". It does not imply complexity or simplicity but does suggest an "advance" from the original (or early ancestral) character(s). Using the terms primitive and advanced in a comparative sense allows us to consider phylogenetic juxtaposition.

I agree with Bruno and Riley that it is better to avoid the loaded term "primitive" - loaded with its associations with the 19th century attitudes to human races and the miscredited "ladder" attitude to the status of groups of organisms. Ancestral implies much more that these organisms were - or were close to - the first representatives of any clade. However, as Riley noted, evolutionary trends are not always in the direction of complexity or novelty and many species are a mixture of traits. Thus it is usually more precise to refer to ancestral features or characteristics rather than regarding whole organisms as ancestral.

Basal or ancestral

Hennig's cladistic approach used a new terminology to highlight that branching points and features that are newly derived at branching points is what matters. His terms were 'plesiomorphic' for the 'ancestral' state and 'apomorphic' for the 'derived' state. Cladists aim to find unique derived conditions that bind all forms derived from a branching point with 'synapomorphies' of unique shared derived characters. "Symplesiomorphies' shared by a species ancestral to the synapomorphic branching points may result in false grouping such as 'Cyclostomes' (defined by absence of lower jaws). However, cyclostomes show now that the rejected clade formed by using symplesiomorphies is now defined by molecular synapomorphies. Unfortunately, the old terminology for this clade is now used again confusing everyone. Simple translations of 'plesiomorph' and 'apomorph' would by 'ancestral' and 'derived'.

There is no general agreement on this. An extinct ancestor with surviving descendants can reasonably be called ancestral. In phylogenetics, the clade that branched nearest the root and is sister to all other clades in a larger clade is called basal, but the term is used more widely as well. Primitive is fair enough for the long-extinct first organisms in a new habitat (e.g. primitive land plants). Early is an alternative for long-extinct groups. However, jellyfish and mammals are exactly the same evolutionary distance from their common ancestor, so unless you have a robust phylogeny I would stick with 'different'!

Always the position of an organism is made based on Phylogenetic analyses. This may be basal or derivated in this context, and to have some traits plesiomorphic, and others derivates. So, would be correct to say basal organism but, however, depend on your root (it is important)

I might suggest "concestor" coined by Richard Dawkins in his excellent book, The Ancestor's Tale: a Pilgrimage to the Dawn of Evolution

https://www.kirkusreviews.com/book-reviews/richard-dawkins/the-ancestors-tale/