Vand-til-land overgang hos tidlige tetrapoder

Vand-til-land-overgangen er en af ​​de vigtigste og mest inspirerende store overgange i hvirveldyrs evolution. Og spørgsmålet om, hvordan og hvornår tetrapoder gik fra vand til land, har længe været en kilde til undren og videnskabelig debat.

Tidlige ideer hævdede, at udtørringsbassiner af vand strandede fisk på land, og at det at være ude af vandet gav det selektive pres til at udvikle flere lemmerlignende vedhæng for at gå tilbage til vandet. I 1990'erne antydede nyopdagede eksemplarer, at de første tetrapoder beholdt mange akvatiske egenskaber, som gæller og en halefinne, og at lemmerne kan have udviklet sig i vandet, før tetrapoder tilpassede sig livet på land. Der er, imidlertid, stadig usikkerhed om, hvornår vand-til-land-overgangen fandt sted, og hvor terrestriske tidlige tetrapoder egentlig var.

Et papir udgivet 25. november i Natur behandler disse spørgsmål ved hjælp af fossile data i høj opløsning og viser, at selv om disse tidlige tetrapoder stadig var bundet til vand og havde akvatiske egenskaber, de havde også tilpasninger, der indikerer en vis evne til at bevæge sig på land. Selvom, de har måske ikke været særlig gode til at gøre det, i hvert fald efter nutidens standarder.

Hovedforfatter Blake Dickson, Ph.d. '20 i Institut for Organismisk og Evolutionær Biologi ved Harvard University, og seniorforfatter Stephanie Pierce, Thomas D. Cabot lektor ved Institut for Organisk og Evolutionær Biologi og kurator for hvirveldyrs palæontologi ved Museum of Comparative Zoology ved Harvard University, undersøgt 40 tredimensionelle modeller af fossil humeri (overarmsknogle) fra uddøde dyr, der bygger bro over vand-til-land-overgangen.

"Fordi fossilregistreringen af ​​overgangen til land i tetrapoder er så dårlig, gik vi til en kilde af fossiler, der bedre kunne repræsentere hele overgangen hele vejen fra at være en fuldstændig vandfisk til en fuldt terrestrisk tetrapod, " sagde Dickson.

To tredjedele af fossilerne kom fra de historiske samlinger på Harvard's Museum of Comparative Zoology, som er hentet fra hele verden. For at udfylde de manglende huller, Pierce nåede ud til kolleger med nøgleeksemplarer fra Canada, Skotland, og Australien. Af betydning for undersøgelsen var nye fossiler, der for nylig blev opdaget af medforfatterne Dr. Tim Smithson og professor Jennifer Clack, University of Cambridge, Storbritannien, som en del af TW:eed-projektet, et initiativ designet til at forstå den tidlige udvikling af landgående tetrapoder.

Forskerne valgte humerusknoglen, fordi den ikke kun er rigelig og velbevaret i fossiloptegnelsen, men det er også til stede i alle sarkopterygier - en gruppe af dyr, som omfatter coelacanth fisk, lungefisk, og alle tetrapoder, inklusive alle deres fossile repræsentanter. "Vi forventede, at humerus ville bære et stærkt funktionelt signal, da dyrene gik fra at være en fuldt funktionel fisk til at være fuldt ud terrestriske tetrapoder, og at vi kunne bruge det til at forudsige, hvornår tetrapoder begyndte at bevæge sig på land, " sagde Pierce. "Vi fandt ud af, at jordiske evner ser ud til at falde sammen med lemmernes oprindelse, hvilket er rigtig spændende."

Humerus forankrer forbenet på kroppen, er vært for mange muskler, og skal modstå meget stress under lemmerbaseret bevægelse. På grund af dette, det rummer en hel del kritisk funktionel information relateret til et dyrs bevægelser og økologi. Researchers have suggested that evolutionary changes in the shape of the humerus bone, from short and squat in fish to more elongate and featured in tetrapods, had important functional implications related to the transition to land locomotion. This idea has rarely been investigated from a quantitative perspective—that is, indtil nu.

When Dickson was a second-year graduate student, he became fascinated with applying the theory of quantitative trait modeling to understanding functional evolution, a technique pioneered in a 2016 study led by a team of paleontologists and co-authored by Pierce. Central to quantitative trait modeling is paleontologist George Gaylord Simpson's 1944 concept of the adaptive landscape, a rugged three-dimensional surface with peaks and valleys, like a mountain range. On this landscape, increasing height represents better functional performance and adaptive fitness, and over time it is expected that natural selection will drive populations uphill towards an adaptive peak.

Dickson and Pierce thought they could use this approach to model the tetrapod transition from water to land. They hypothesized that as the humerus changed shape, the adaptive landscape would change too. For instance, fish would have an adaptive peak where functional performance was maximized for swimming and terrestrial tetrapods would have an adaptive peak where functional performance was maximized for walking on land. "We could then use these landscapes to see if the humerus shape of earlier tetrapods was better adapted for performing in water or on land" said Pierce.

"We started to think about what functional traits would be important to glean from the humerus, " said Dickson. "Which wasn't an easy task as fish fins are very different from tetrapod limbs." In the end, they narrowed their focus on six traits that could be reliably measured on all of the fossils including simple measurements like the relative length of the bone as a proxy for stride length and more sophisticated analyses that simulated mechanical stress under different weight bearing scenarios to estimate humerus strength.

"If you have an equal representation of all the functional traits you can map out how the performance changes as you go from one adaptive peak to another, " Dickson explained. Using computational optimization the team was able to reveal the exact combination of functional traits that maximized performance for aquatic fish, terrestrial tetrapods, and the earliest tetrapods. Their results showed that the earliest tetrapods had a unique combination of functional traits, but did not conform to their own adaptive peak.

"What we found was that the humeri of the earliest tetrapods clustered at the base of the terrestrial landscape, " said Pierce. "indicating increasing performance for moving on land. But these animals had only evolved a limited set of functional traits for effective terrestrial walking."

The researchers suggest that the ability to move on land may have been limited due to selection on other traits, like feeding in water, that tied early tetrapods to their ancestral aquatic habitat. Once tetrapods broke free of this constraint, the humerus was free to evolve morphologies and functions that enhanced limb-based locomotion and the eventual invasion of terrestrial ecosystems

"Our study provides the first quantitative, high-resolution insight into the evolution of terrestrial locomotion across the water-land transition, " said Dickson. "It also provides a prediction of when and how [the transition] happened and what functions were important in the transition, at least in the humerus."

"Bevæger sig fremad, we are interested in extending our research to other parts of the tetrapod skeleton, " Pierce said. "For instance, it has been suggested that the forelimbs became terrestrially capable before the hindlimbs and our novel methodology can be used to help test that hypothesis."

Dickson recently started as a Postdoctoral Researcher in the Animal Locomotion lab at Duke University, but continues to collaborate with Pierce and her lab members on further studies involving the use of these methods on other parts of the skeleton and fossil record.