{"id":27163,"date":"2023-03-17T10:48:02","date_gmt":"2023-03-17T10:48:02","guid":{"rendered":"https:\/\/research.reading.ac.uk\/research-blog\/?p=27163"},"modified":"2023-03-17T10:56:10","modified_gmt":"2023-03-17T10:56:10","slug":"how-fish-evolved-to-walk-and-in-one-case-turned-into-humans","status":"publish","type":"post","link":"https:\/\/research.reading.ac.uk\/research-blog\/2023\/03\/17\/how-fish-evolved-to-walk-and-in-one-case-turned-into-humans\/","title":{"rendered":"How fish evolved to walk \u2013 and in one case, turned into humans"},"content":{"rendered":"

When you think about human evolution, there\u2019s a good chance you\u2019re imagining chimpanzees exploring ancient forests or early humans daubing woolly mammoths on to cave walls. But we humans, along with bears, lizards, hummingbirds and\u00a0Tyrannosaurus rex<\/em>,\u00a0are actually lobe-finned fish<\/a>.<\/p>\n

\"3D
3D rendering of the tiktaalik, an extinct walking fish.<\/figcaption><\/figure>\n

It might sound bizarre but the evidence is in our\u00a0genes<\/a>,\u00a0anatomy<\/a>\u00a0and in\u00a0fossils<\/a>. We belong to a group of animals called land-dwelling\u00a0sarcopterygians<\/a>, but vast amounts of evolutionary change have obscured our appearance.<\/p>\n

We think of fish as expert swimmers, but in fact they have evolved the ability to \u201cwalk\u201d\u00a0at least five times<\/a>. Some species\u00a0pull themselves forward<\/a>\u00a0using well developed fore-fins, while others \u201cwalk\u201d along the ocean floor.<\/p>\n

Our sarcopterygian ancestor evolved lungs and other air-breathing mechanisms, bony limbs, and a stronger spinal column before venturing on to land. These adaptations were useful not only in aquatic environments but allowed our ancestors to explore land \u2013 they were \u201cpre-adaptations\u201d for life on land.<\/p>\n

The water-to-land transition was one of the\u00a0most significant events in vertebrate evolution<\/a>. It may have started as a way to escape from predators, but the landscape our ancestors discovered was already\u00a0rich with plants<\/a>\u00a0such as mosses, horsetails and ferns, as well as arthropods (millipedes)\u00a0which had colonised land<\/a>\u00a0millions of years earlier.<\/p>\n

We are not alone<\/h3>\n

Walking independently evolved several times in fish, making it an example of evolutionary convergence (similar traits that evolve independently,\u00a0like wings in bats and birds<\/a>). The evolution of walking in fish is rare though. There are more than 30,000 species of fish as we know them today (not in the evolutionary sense), of which only a handful can \u201cwalk\u201d.<\/p>\n

Sarcopterygians differ from other types of fish in several important ways. For example, our fins (limbs) have\u00a0bony supports and muscular lobes<\/a>\u00a0that allow us to move on land.<\/p>\n

This adaptation is thought to have been crucial for tetrapod (amphibians, mammals, reptiles and birds) evolution during our transition from water to land in the Late Devonian period, some 375 million years ago. Many of the genes involved in forming limbs and digits in tetrapods are also found in\u00a0water-bound sarcopterygians like lungfish<\/a>, which indicates these traits evolved in our ancient common ancestor.<\/p>\n

\"Coelacanth
Coelacanth fish still exist in tropical seas.<\/figcaption><\/figure>\n

We don\u2019t know which species this ancestor was, but it probably\u00a0looked similar to the coelacanth<\/a>, which has a rich fossil record and is a \u201cliving fossil\u201d that today inhabits the West Indian Ocean and Indonesia.<\/p>\n

Walking sarcopterygian fish are either extinct, like\u00a0Tiktaalik<\/em><\/a>, or so highly evolved that we don\u2019t recognise them as fish any longer (tetrapods).<\/p>\n

One example of a living fish that walks is the mudskipper (of the\u00a0Oxudercidae family<\/a>). These fish live in mangrove swamps and tidal flats and use their pectoral fins to walk on land. These fins help them to escape from aquatic predators, forage for food (they consume organic material in mud), and even interact with each other on land by finding mates.<\/p>\n

Another example is the walking catfish (Clarias batrachus<\/em>), which uses its pectoral fins to travel over land, helping it escape from drying ponds and find new habitats.<\/p>\n

How did genes related to walking first evolve?<\/h3>\n

The little skate (Leucoraja erinacea<\/em>) is a cartilaginous fish related to rays and sharks (as opposed to bony fish, including sarcopterygians). It is another fish that \u201cwalks\u201d underwater on fins like legs, mimicking the movements of land-based animals.<\/p>\n

The little skate is of great interest to scientists researching the evolution of locomotion because it evolved fin-based walking independently from sarcopterygians. However, until now, the genetics behind the walking of the little skate was difficult to study due to a lack of quality data.<\/p>\n