Periodically, life on Earth finds itself on a nearly vacant stage, facing an extraordinary opportunity. A remarkable transformation occurs in the atmosphere, oceans, or within organisms themselves, leading to the emergence of a completely new world. These pivotal moments in the history of life provide scientists with a captivating glimpse into evolution.
Recently, a group of researchers from the University of Chicago undertook a study to investigate the evolutionary journey of bivalves, which encompass clams, mussels, scallops, and oysters, among others. Their focus was on the Cambrian Explosion, a period of rapid evolution. Surprisingly, while numerous other lineages swiftly diversified and developed a wide range of forms and functions, the bivalves fell behind. It is possible that this delay was due to their protracted process of evolving a specific adaptation required for their prosperity.
The findings of this study have profound implications for our understanding of evolution and the consequences of extinctions, according to the scientists involved.
Shell and high water
Around 500 million years ago, life on Earth experienced a remarkable event known as the Cambrian Explosion. During this period, a multitude of new life forms emerged, contributing to the diverse array of species we observe today.
Among these organisms were bivalves, which are characterized by their hard, twin-shelled bodies and their habitat on the seafloor. Intrigued by the bivalves’ evolution in a relatively empty ocean with a fresh body design, a team of scientists embarked on a comprehensive study to document their rise.
The research team, consisting of experts like Stewart Edie from the Smithsonian’s National Museum of Natural History, Katie Collins from the U.K.’s Natural History Museum, and Sharon Zhou, an undergraduate student at the University of Chicago, meticulously examined the fossil record. They carefully analyzed each known fossil species to unravel how bivalves developed new forms and lifestyles, such as burrowing into sediment or attaching themselves to rocks. Zhou explained that the shell’s shape, for instance, could indicate whether a bivalve was adapted for burrowing, as it would become elongated and slender.
Through their meticulous efforts, the researchers uncovered a surprising revelation about the bivalves’ evolution.
One might assume that these organisms swiftly capitalized on their new body design, propelling themselves to success and biological prominence. However, that wasn’t the case. The bivalves evolved at a slower pace compared to other groups that originated during the same period. David Jablonski, a geophysical sciences professor at the University of Chicago and co-corresponding author of the paper, expressed his surprise, stating, “It’s kind of amazing they made it through at all.” Even when the bivalves began to diversify approximately 40 million years later, they never exhibited an explosive increase in species or ecological niches.
To rule out the possibility of a biased interpretation due to gaps in the fossil record, the researchers conducted various tests and computer simulations. The results indicated that the slow start observed in the bivalves’ evolution was likely an accurate representation. Although fossils from that era are challenging to find and identify due to rock transformations, Collins explained that the simulations would need to be highly extreme to alter the observed pattern in the rocks.
The reason behind the bivalves’ lag remains uncertain, but one hypothesis suggests that they had not yet developed a crucial organ: an enlarged gill for filtering plankton from water, a common feature among many modern bivalves. By the time they acquired this adaptation, the seafloor had become densely populated. Jablonski drew an analogy, stating, “If you show up early to the dance floor, you can do whatever you want, but if you show up late, it restricts the range of moves.”
Despite their delayed progress, bivalves not only survived but thrived to this day. Jablonski highlighted the significance of this finding, stating, “It tells us there’s more than one pathway to success, even when you are starting at the very beginning of multicellular life.”
Scientists are keenly interested in documenting these evolutionary accounts as they provide insights into how life adapts and diversifies following significant disruptions or extinctions. The researchers plan to further investigate how bivalves responded to extinctions over time to identify any recurring patterns.
Jablonski emphasized the relevance of understanding the repopulation process after extinctions, particularly in the context of the ongoing major extinction event the Earth is currently experiencing.
The study also served as a valuable learning experience for Sharon Zhou, an undergraduate at the University of Chicago. Initially intending to pursue mathematics, she became captivated by evolutionary biology after enrolling in a course to fulfill her university’s science requirements. Zhou spent several years working in Jablonski’s lab and now plans to pursue graduate.
Source: University of Chicago