Unlike real intelligent design, evolution has no plan, no foresight and no ability to review progress and start again. It is a non-sentient, reactive process, constrained by what already exists, and able to respond to changing conditions only by favouring advantageous variations in the inherited material available to it. The result is that future generations always carry traces of earlier generations — not merely for a few years, but for millions of years.
This simple consequence of descent with modification explains why living organisms can be grouped into clades, and why the descendants of earlier taxa remain members of those taxa. These nested hierarchies are themselves confirmation of the Theory of Evolution because they are exactly what the theory predicts. Conversely, they are evidence against intelligent design, which, if it were real, should show no such inherited constraint, but rather repeated innovation, clean design solutions and optimal responses to environmental change.
For a good example, look no further than the familiar sideways scuttle of a shore crab (Carcinus maenas). This distinctive form of locomotion is characteristic of true crabs, although not universal among them, and a research team led by Associate Professor Yuuki Kawabata of the Graduate School of Integrated Science and Technology, Nagasaki University, Japan, has shown that it probably arose once, in a common ancestor, around 200 million years ago. Their findings have just been published as a Reviewed Preprint in eLife.
Background^ What Are “True Crabs”? “True crabs” are the crustaceans classified in the infraorder Brachyura, a name meaning “short tail”. Their most obvious feature is the compact, flattened body with the abdomen folded tightly underneath the thorax, giving them the familiar crab-like shape.The publication is accompanied by a press release by eLife.
They belong to the phylum Arthropoda — animals with jointed limbs, segmented bodies and a hard external skeleton — and, more specifically, to the crustacean class Malacostraca. Within that class they are members of the order Decapoda, the “ten-footed” crustaceans, which also includes lobsters, crayfish, prawns and shrimps.
This means that true crabs are not isolated curiosities, but one highly successful branch of a much larger arthropod family tree. Their relatives include other familiar marine crustaceans, but not all animals called “crabs” are true crabs. King crabs, porcelain crabs and hermit crabs, for example, belong to a different decapod group, the Anomura. They look crab-like because evolution has repeatedly produced similar body plans in related crustaceans — a process known as carcinisation.
That repeated evolution of crab-like forms is itself an elegant example of evolutionary constraint. Natural selection does not design from scratch; it modifies what is already present. In decapod crustaceans, a broad, compact body, reduced tail and strong walking limbs have evolved more than once because they can be useful in similar ecological settings, especially on the sea floor, in crevices, among rocks, and in other predator-rich habitats.
True crabs have been especially successful. There are thousands of known species, living not only in the sea but also in freshwater and on land. Their diversity includes shore crabs, edible crabs, fiddler crabs, spider crabs, swimming crabs and many others. The common shore crab, Carcinus maenas, is therefore just one member of a large and ancient evolutionary radiation.
The sideways walk of many true crabs is part of this inherited history. It is not an isolated trick invented separately in each species, but a trait that appears to have arisen deep in the crab lineage and was then retained, modified or occasionally lost in descendant groups. Once again, the pattern is not one of fresh design, but of descent with modification.
Study suggests crabs’ iconic sideways walk evolved from common ancestor
New findings suggest sideways walking in crabs is a rare but innovative trait originating from a common ancestor, and provide a framework for understanding how animal locomotion diversifies and persists over time.
Researchers have provided new insights into the evolutionary origin of sideways walking in crabs.
Their study, published today as a Reviewed Preprint in eLife, presents the largest comparative dataset on crab locomotion to help understand the origins of the animals’ iconic walk, tracing it back to a common ancestor around 200 million years ago. eLife’s editors say the work is valuable, with largely convincing evidence, and will be of interest to others studying animal locomotion.
Sideways walking is a defining feature of ‘true crabs’ (Brachyura) – the largest of the crab decapod groups. Among other benefits, this mode of travel may be particularly useful for escaping from predators as it makes the crabs’ escape direction unpredictable.
Sideways locomotion may have contributed significantly to the ecological success of true crabs. There are around 7,904 species of true crabs, far exceeding that of their sister group, Anomura, or their closest relatives, Astacidea; they have colonised diverse habitats around the world, including terrestrial, freshwater and deep-sea environments; and their crab-like body shape has evolved repeatedly over time in a phenomenon known as carcinisation.
Despite the rich information available on true crabs, data concerning their locomotor behaviours are sparse. Although most true crab species use sideways locomotion, there are some groups that walk forwards, which raises some interesting questions. When did their sideways locomotion originate, how many times over the years did it evolve, and how many times did it revert?
Associate Professor Yuuki Kawabata, senior author.
Graduate School of Integrated Science and Technology
Nagasaki University, Japan.
To address these questions, Kawabata and colleagues first carried out behavioural analyses of 50 true crab species. Using a standard video camera, they recorded for 10 minutes the movements of each species in plastic circular arenas that matched their native environment. Due to logistical constraints, the movements of one representative crab from each species were recorded.
They then combined their behavioural analysis with data extracted from a recently published crab phylogeny* – a study that reconstructed the evolutionary history of Brachyura using sequences of 10 genes for 344 species across most major lineages of true crabs. As their behavioural dataset did not always match the species included in the phylogeny, the team reduced the evolutionary tree to 44 genera (a rank that sits in between a species and a family), five families and one superfamily, allowing closely related groups to represent the observed species when the same species were unavailable.
Of all 50 species covered, the team classified 35 as sideways movers and 15 as forward movers. Their analyses revealed that the change to sideways walking occurred only once from a single, forward-walking ancestor at the base of Eubrachyura (a group comprising more advanced crab species) and then remained highly conserved across true crabs.
This single event contrasts starkly with carcinisation, which has occurred repeatedly across decapod species. This highlights that while body shapes may converge multiple times, behavioural changes such as sideways walking can be rare.
Associate Professor Yuuki Kawabata.
The single origin and diversity of Eubrachyura are consistent with the idea that sideways walking acted as a key innovation that contributed to the ecological success of true crabs. One likely adaptive advantage of this mode of travel is the ability to move rapidly at similar speeds in both lateral directions to allow for easier escape from predators. Despite this advantage, it has been difficult to evolve across the animal kingdom, possibly because it affects other behaviours such as burrowing, mating and foraging. The authors say it therefore represents a unique event that has occurred only in true crabs, and potentially crab spiders and leafhopper nymphs.
Finally, they note that the innovation of sideways walking may not be the only process contributing to the evolutionary diversification of true crabs. External factors, such as the ecological opportunity provided by mass extinction, are also critical for evolutionary diversification. Indeed, their analysis suggests that the origin of true crabs’ sideways walk falls around 200 million years ago (the earliest Jurassic, immediately post–Triassic–Jurassic extinction). This period is marked by Pangaean rifting, expansion of shallow-marine habitats and the early Mesozoic Marine Revolution, all of which typically increased ecological opportunity.
To disentangle the relative roles of innovation and environmental change, we need further analyses of trait-dependent diversification, fossil-informed timelines and performance tests that link true crabs’ sideways movement to adaptive advantages. These current results highlight that sideways locomotion in true crabs is a rare but innovative trait that may have contributed to their ecological success. Such innovations can open new adaptive opportunities and yet remain constrained by phylogenetic history and ecological contexts. With direct behavioural observations and a phylogenetic framework, this work expands our understanding of how modes of travel in animals diversify and persist through evolutionary time.
Associate Professor Yuuki Kawabata.
Yuuki Kawabata carried out this study alongside co-first authors Junya Taniguchi, Tsubasa Inoue and Kano Kohara, from the Kawabata Laboratory. Additional authors include Jung-Fu Huang, National Kaohsiung University of Science and Technology, Taiwan; Atsushi Hirai, Susami Crustacean Aquarium, Wakayama, Japan; Nobuaki Mizumoto, Auburn University, Alabama, US; and Fumio Takeshita, Kitakyushu Museum of Natural History & Human History, Japan.
Publication:
What Kawabata and colleagues have shown is exactly what evolutionary biology predicts: a distinctive feature appearing deep in a lineage, then being inherited, modified, retained, or occasionally lost by its descendants. True crabs did not each need to be separately equipped with a sideways gait by some invisible designer. The pattern is far simpler, and far more powerful: one ancestral innovation, passed down through descent with modification.
That is why taxonomy works. Living things fall into nested hierarchies because they have histories. Crabs are arthropods, crustaceans, malacostracans, decapods and brachyurans, not because someone designed a filing system for them, but because each level represents ancestry and inheritance. They carry their evolutionary past in their bodies, their limbs, their genes and even in the way they move across the shore.
Creationism and its Intelligent Design offshoots have no comparable explanation. A designer free to start again would not be constrained by ancestral body plans, inherited limb arrangements, folded abdomens, modified appendages and historical compromises. Those are not the marks of unconstrained design; they are the marks of descent from earlier forms.
So, the sideways scuttle of a common shore crab is more than a familiar seaside sight. It is a small, everyday reminder that evolution has no need of foresight, planning or perfection. It only needs variation, inheritance, selection and time — and over hundreds of millions of years, even a walk along the beach can become a record of deep evolutionary history.
Advertisement
Amazon
Amazon
Amazon
Amazon
Amazon
Amazon
Amazon
Amazon
Amazon
Amazon
Amazon
Amazon
All titles available in paperback, hardcover, ebook for Kindle and audio format.
Prices correct at time of publication. for current prices.
No comments:
Post a Comment
Obscene, threatening or obnoxious messages, preaching, abuse and spam will be removed, as will anything by known Internet trolls and stalkers, by known sock-puppet accounts and anything not connected with the post,
A claim made without evidence can be dismissed without evidence. Remember: your opinion is not an established fact unless corroborated.