In a unique analysis that provides a glimpse into the origin stories of new animal traits, a pair of research studies led by University of California San Diego and Harvard University scientists has traced the evolutionary adaptations of octopus and squid sensing capabilities. The studies, featured on the cover of the April 13 issue of Nature, reveal evolutionary links to human brain receptors.

Researchers with Ryan Hibbs’ newly established laboratory in the School of Biological Sciences at UC San Diego (formerly based at the University of Texas Southwestern Medical Center) and Nicholas Bellono’s lab at Harvard analyzed octopuses and squids, animals known as cephalopods, through a comprehensive lens that spanned atomic-level protein structure to the entire functional organism. They focused on sensory receptors as a key site for evolutionary innovation at the crossroads of ecology, neural processing and behavior.

By looking at the way octopuses and squids sense their marine environments, the researchers discovered new sensory receptor families and determined how they drive distinct behaviors in the environment. With cryo-electron microscopy technology, which uses cryogenic temperatures to capture biological processes and structures in unique ways, they showed that adaptations can help propel new behaviors.

“Cephalopods are well known for their intricate sensory organs, elaborate nervous systems and sophisticated behaviors that are comparable to complex vertebrates, but with radically different organization,” said Hibbs, a professor in the Department of Neurobiology. Hibbs brings expertise on the structure of a family of proteins in humans that mediate communication between brain neurons and other areas such as between neurons and muscle cells. “Cephalopods provide striking examples of convergent and divergent evolution that can be leveraged to understand the molecular basis of novelty across levels of biological organization.”

In one Nature study, the research teams described for the first time the structure of an octopus chemotactile (meaning chemical and touch) receptor, which octopus arms use for taste-by-touch exploration. These chemotactile receptors are similar to human brain and muscle neurotransmitter receptors, but are adapted through evolution to help evaluate possible food sources in the marine environment.

“In octopus, we found that these chemotactile receptors physically contact surfaces to determine whether the animal should eat a potential food source or reject it,” said Hibbs. “Through its structure, we found that these receptors are activated by greasy molecules, including steroids similar to cholesterol. With evolutionary, biophysical and behavioral analyses, we showed how strikingly novel structural adaptations facilitate the receptor’s transition from an ancestral role in neurotransmission to a new function in contact-dependent chemosensation of greasy environmental chemicals.”

The second Nature study focused on squid and their wholly different ambush strategy for capturing food. The researchers combined genetics, physiology and behavioral experiments to discover a new class of ancient chemotactile receptors and determined one structure within the class. They also conducted an evolutionary analysis to link adaptations in squid receptors to more elaborate expansions in octopus. They were then able to place chemotactile and ancestral neurotransmitter receptors on an evolutionary timeline and described how evolutionary adaptations drove the development of new behaviors.

“We discovered a new family of cell surface receptors that offer a rare lens into the evolution of sensation because they represent the most recent and only functionally tractable transition from neurotransmitter to environmental receptors across the entire animal kingdom,” said Hibbs. “Our structures of these unique cephalopod receptors lay a foundation for the mechanistic understanding of major functional transitions in deep evolutionary time and the origin of biological novelty.”

Hibbs says the pair of new studies offers an excellent example of how curiosity in interesting creatures can lead to insights important for all of biology, namely how proteins — life’s building blocks — adapt to mediate new functions and behaviors.

“These studies are a great example of what being a scientist is all about — wonder, exploration and understanding how things work,” he said.

Video: https://youtu.be/WC4h_XIioAw

However, unlike their neighbors, the Xiongnu never developed a writing system, and consequently historical records about the Xiongnu have been almost entirely written and passed down by their rivals and enemies. Such accounts, largely recorded by Han Dynasty chroniclers, provide little useful information on the origins of the Xiongnu, their political rise, or their social organization. Although recent archaeogenetics studies have now traced the origins of the Xiongnu as a political entity to a sudden migration and mixing of disparate nomadic groups in northern Mongolia ca. 200 BCE, such findings have raised more questions than answers.

To better understand the inner workings of the seemingly enigmatic Xiongnu empire, an international team of researchers at the Max Planck Institutes for Evolutionary Anthropology (MPI-EVA) and Geoanthropology (MPI-GEO), Seoul National University, the University of Michigan, and Harvard University conducted an in-depth genetic investigation of two imperial elite Xiongnu cemeteries along the western frontier of the empire: an aristocratic elite cemetery at Takhiltyn Khotgor and a local elite cemetery at Shombuuzyn Belchir.

“We knew that the Xiongnu had a high degree of genetic diversity, but due to a lack of community-scale genomic data it remained unclear whether this diversity emerged from a heterogeneous patchwork of locally homogenous communities or whether local communities were themselves genetically diverse,” explains Juhyeon Lee, first author of the study and PhD student at Seoul National University. “We wanted to know how such genetic diversity was structured at different social and political scales, as well as in relation to power, wealth, and gender.”

The rise of a multiethnic empire

Researchers found that individuals within the two cemeteries exhibited extremely high genetic diversity, to a degree comparable with that found across the Xiongnu Empire as a whole. In fact, high genetic diversity and heterogeneity was present at all levels — across the empire, within individual communities, and even within individual families — confirming the characterization of the Xiongnu Empire as a multiethnic empire. However, much of this diversity was stratified by status. The lowest status individuals (interred as satellite burials of the elites, likely reflecting a servant status) exhibited the highest genetic diversity and heterogeneity, suggesting that these individuals originated from far-flung parts of the Xiongnu Empire or beyond. In contrast, local and aristocratic elites buried in wood-plank coffins within square tombs and stone ring graves exhibited lower overall genetic diversity and harbored higher proportions of eastern Eurasian ancestries, suggesting that elite status and power was concentrated among specific genetic subsets of the broader Xiongnu population. Nevertheless, even elite families appear to have used marriage to cement ties to newly incorporated groups, especially at Shombuuzyn Belchir.

“We now have a better idea of how the Xiongnu expanded their empire by incorporating disparate groups and leveraging marriage and kinship into empire building,” says senior author Dr. Choongwon Jeong, Associate Professor of Biological Sciences at Seoul National University.

Powerful women in Xiongnu society

A second major finding was that high status Xiongnu burials and elite grave goods were disproportionately associated with women, corroborating textual and archaeological evidence that Xiongnu women played especially prominent political roles in the expansion and integration of new territories along the empire’s frontier. At the aristocratic elite cemetery of Takhiltyn Khotgor, researchers found that the elite monumental tombs had been built for women, with each prominent woman flanked by a host of commoner males buried in simple graves. The women were interred in elaborate coffins with the golden sun and moon emblems of Xiongnu imperial power and one tomb even contained a team of six horses and a partial chariot. At the nearby local elite cemetery of Shombuuzyn Belchir, women likewise occupied the wealthiest and most elaborate graves, with grave goods consisting of wooden coffins, golden emblems and gilded objects, glass and faience beads, Chinese mirrors, a bronze cauldron, silk clothing, wooden carts, and more than a dozen livestock, as well as three objects conventionally associated with male horse-mounted warriors: a Chinese lacquer cup, a gilded iron belt clasp, and horse tack. Such objects and their symbolism convey the great political power of the women.

“Women held great power as agents of the Xiongnu imperial state along the frontier, often holding exclusive noble ranks, maintaining Xiongnu traditions, and engaging in both steppe power politics and the so-called Silk Road networks of exchange,” says Dr. Bryan Miller, project archaeologist and Assistant Professor of Central Asian Art & Archaeology at the University of Michigan.

Children in Xiongnu society

Genetic analysis also provided rare insights into the social roles of children in Xiongnu society. “Children received differential mortuary treatment depending upon age and sex, giving clues to the ages at which gender and status were ascribed in Xiongnu society,” says senior author Dr. Christina Warinner, Associate Professor of Anthropology at Harvard University and Group Leader at the Max Planck Institute for Evolutionary Anthropology. Researchers found, for example, that although adolescent Xiongnu boys as young as 11-12 years old were buried with a bow and arrows, in a manner resembling that of adult males, younger boys were not. This suggests that the gendered social roles of hunter and warrior were not ascribed to boys until late childhood or early adolescence.

The legacy of the Xiongnu today

Although the Xiongnu empire ultimately disintegrated in the late 1st century CE, the findings of the study point to the enduring social and cultural legacy of the Xiongnu. “Our results confirm the long-standing nomadic tradition of elite princesses playing critical roles in the political and economic life of the empires, especially in periphery regions — a tradition that began with the Xiongnu and continued more than a thousand years later under the Mongol Empire,” says Dr. Jamsranjav Bayarsaikhan, project archaeologist and Mongolian Archaeology Project: Surveying the Steppes (MAPSS) project coordinator at the Max Planck Institute for Geoanthropology. “While history has at times dismissed nomadic empires as fragile and short, their strong traditions have never been broken.”