Ive always wondered this: has a species have an evolutionary history where they originated in land but adapted to living in water and then adapted to living on land again?
It’s strongly theorized that turtle ancestors went from land to sea and then back to land. However, it’s tough to completely pin down because turtles have a very bizarre evolutionary history. For example there are proto turtles with shells but without beaks but also proto turtles with beaks but without shells suggesting that either beaks or shells evolved twice and one of those lines died out.
The evolutionary tree, maybe more accurately the phylogenetic tree, wouldn't loop as it is derived from genetics in modern times. Our ancestors' attempts at making evolutionary trees from morphology (can't fault them for not sequencing DNA) have got a lot of things wrong because seemingly relating creatures were not at all. Convergent evolution being a major reason for stuff they got wrong, making morphology a poor approximation of phylogeny.
Even if 2 distinct species eons apart ended up literally identical and sexually compatible thru some cosmic luck, they wouldn't show up as a loop since we could tell how far apart they are by looking at how mutated some useless and asexually transmitted gene is (mitochondria DNA provides that for eukaryotes, say cytochrome oxidase I gene). The key part is that the gene doesn't receive any evolutionary pressure, so that it only drifts thru random mutations (considering they can't provide a reproductive advantage/increase "fitness"). If you know the mutation rate (which we do), you can date how far back a common ancestor goes between two species' by basically running a diff script on their COI gene sequences.
Considering every known form of life descends from a single ancestor (LUCA [1]), a tree (or root system) analogy is quite apt.
On a large scale, there can not be loops, unless we accept the existence of time travel. But there are indeed some complications that make the concept of a tree of life surprisingly subtle:
- ring species
- species complexes
- hybrid species (turning the tree into a Directed Acyclic Graph)
- horizontal gene transfer via plasmids (between bacteria) or viruses
Not exactly what you’re asking but whales used to be land mammals.
> It is believed that modern-day whales evolved from land-based animals about 55 million years ago. These land-based mammals are believed to be hoofed mammals, sharing a common ancestry with even-toed ungulates such as the cow and the deer.
> Evolution of Whales
Whales started their journey as all other organisms have, as single-celled bacteria.
An evolutionary picture of the whales can be broken down as follows:
- 3.8 billion years ago: The first single-celled organisms appeared (Bacteria)
- 3 billion years ago: Viruses (also single-celled organisms) became present
- 2 billion years ago: Eukaryotic cells are present. These are cells that contain organelles, or tiny organ-like structures
- 1.5 billion years ago: Eukaryotic cells evolved three ways. These cells evolved into the ancestors of plants, animals, and fungi
- 900 million years ago: The first multicellular structures became present
- 800 million years ago: The animal strain of organisms undergoes its first split and continues into basic marine organisms such as sponges
- 540 million years ago: The first chordates or animals with backbones are present
- 530 million years ago: The first true vertebrate or boned organism is present
- 500 million years ago: Animals first started exploring the land
- 417 million years ago: Lungfish became present. Lungfish are the first organisms to breathe both on land and in the water with both lungs and gills
- 397 million years ago: The first tetrapods or four-legged species are present
- 340 million years ago: Amphibians branch off from the other tetrapods
- 310 million years ago: The remaining tetrapods split into what will be reptiles, birds and dinosaurs, and mammals
- 200 million years ago: A mass extinction occurred and warm-blooded proto-mammals developed
- 140 million years ago: Placental mammals also known as eutherians are present
- 105-85 million years ago: The placental mammals split into four major groups, including laurasiatheres, which will contain the whale species.
- 65 million years ago: The greatest extinction event so far wipes out the dinosaurs providing more potential for mammals to colonize the planet
- 50 million years ago: Artiodactyls pakicitus, a mammal, resembling a wolf and tapir mix with cloven hooves begins evolving into what we know as whales
- 47 million years ago: Early forms of whales live in shallow seas, returning to land to mate and give birth
- 35-45 million years ago: The first fully aquatic whale is present (Basilosaurus)
Modern-day whales are believed to have moved into the oceans around the Tethys Sea, now the Mediterranean Sea and Asia.
I very much doubt you can pinpoint the date of the appearance of viruses, let alone a full 800 million years after the appearance of bacteria. That site is a click-farm operation.
It's interesting to look at the primary literature, because then you realize how hard it is to sort these questions out and how many uncertainties there are, because there are a lot of different extinct lineages that might or might not have led to whales or hippos.
Here's the general group people seem to be fairly sure led to hippos; the ones that led to whales might have been an earlier group that gave rise to this group, or perhaps one of these, or perhaps not all whales share a 'root whale':
The actual kind of data they use to make these judgements is mostly bone fragments from fossils, as comparing modern DNA of hippos and whales can only tell you so much. Here's an example study from 2003 on the hippo lineage:
Maybe tortoises. There’s a wide variety of ancestors and related animals that lived in the sea, in freshwater, on land. It’s difficult to trace an exact lineage, but they have a lot of cousins in the water and on land.