The ocean is the cradle of life. According to the history of biological evolution, the emergence of mammals has gone through a long process from the origin of life on earth to the birth of ancient fish, and then some fish from the ocean to the land, and began to appear amphibians and even truly out of the water environment, after which a part of the amniotic animals continued to evolve, and mammals were born. It took about 300 million years from the time fish went on land to the appearance of the first mammals.
Among mammals is a special group known as "sea mammals" – marine mammals. Upon disembarkation, its different branches are independently reintegrated from land to the sea, dependent on marine resources for subsistence or completely living in the ocean, and a few live in fresh water.
Why do these mammals, who have struggled to climb from the water to land, "enter the water twice"? How does it evolve and adapt to return to water from land? Recently, the marine mammal research team of the Institute of Deep-sea Sciences and Engineering of the Chinese Academy of Sciences, together with Northwestern Polytechnical University, Qingdao Huada Gene Research Institute and other scientific research institutions, published an article in the Proceedings of the American Academy of Sciences, revealing the important genetic mechanism of marine mammals adapting to the marine environment and their convergent evolution.
Species evolution is driven by climate and environment
Marine mammals refer to mammals adapted to the aquatic environment, and there are 129 extant species. They are marine fetal mammals, lung respiration, constant body temperature, streamlined and forelimbs specialized into finned vertebrates, including cetaceans, pinnipeds, manatees, polar bears and sea otters, as well as a small part of the living in fresh water, such as the white dolphin inhabiting the middle and lower reaches of the Yangtze River in China.
Marine mammals have some common characteristics that adapt to the aquatic environment, such as streamlined body, limbs finning or degeneration, developed hearing, deterioration of smell and taste, thickening of body fat, tolerance to hypoxia, increased tolerance to pressure and increased ability to regulate osmotic pressure.
According to Darwin's theory of evolution, only the fittest survive. The evolution of species on Earth is a concrete embodiment of the adaptive evolution of animals to the environment, mainly driven by climate and environmental changes.
"The evolutionary history of marine mammals is a classic case of survival of the fittest." The corresponding author of the paper told the Science and Technology Daily reporter.
The amazing evolutionary history of the different branches of marine mammals can be told in the Cretaceous period.
At the end of the Cretaceous Period 65 million years ago, the fifth mass extinction event occurred in the history of the Earth, and about 75%-80% of species went extinct. Some researchers believe that the impact of an asteroid on the earth changed the earth's environment at that time, resulting in the extinction of a large number of large land animals, especially dinosaurs. In order to avoid the harsh terrestrial environment, some of the ancestors of the proboscis order entered the ocean 64 million years ago and became amphibian mammals, which in turn formed the current manatee.
Cetaceans originated from the Paleocene-Eocene extreme heat event about 55.5 million years ago. According to Bergman's law, it was the active period of the Earth's plate movement, and the release of large amounts of carbon from the Earth's crust into the air caused the temperature of the atmosphere to rise rapidly, driving the mammals on Earth to evolve in the direction of small sizes and spread around in search of new habitats. The study found that the ancestors of cetaceans were only about 0.5 meters long, and they began the evolutionary process from land to sea in order to avoid high temperatures.
The middle and late Eocene was a period of continuous decline in the Earth's temperature, and the trend of continuous global temperature decline was interrupted in the Oligocene. Temperatures plummeted by 8.2°C over 400,000 years in the early Oligocene, and it was during this period of temperature dip that toothed whales and baleen whales diverged. After that, the Earth entered a period of stable temperature fluctuations from 32.5 million years ago to 25.5 million years ago. The ancestors of pinnipeds originated from this period of steady oscillation. At the same time, this period was also a period of evolution of the diversity of large carnivores and mammals. The study found that polar bears and sea otters have a shorter evolutionary history, beginning to differentiate only about 500,000 to 1 million years ago, and are the youngest marine mammals. Although they retain many of the characteristics of terrestrial mammals, they are mainly dependent on the ocean for survival and are therefore classified as marine mammals.
Dual regulation achieves constant body temperature
The research team conducted this study through whole genome sequencing. In order to maximize the biological commonalities contained in each taxon of marine mammals, they sequenced and assembled the whole genome of 17 marine mammal species from the family level of the natural taxonomy of animals, and reconstructed the most comprehensive marine mammal phylogene tree ever based on whole genome data.
Not only that, in order to analyze the molecular adaptation mechanism of marine mammals from land to the sea, the scientific research team made a large number of comparisons with the whole genome of marine mammals and the whole genome of their terrestrial-related species, and further analyzed and explored the molecular adaptation mechanism of cetaceans, pinnipeds and manatees from land to the ocean from the three main branches of marine mammals, such as genome evolution, gene evolution, and non-coding conservative elements.
"We found that although marine mammals of different branches had completely different ancestors and different evolutionary processes, these animals underwent consistent changes in the genes related to body temperature maintenance, body size, hypoxia tolerance, echo localization, deep diving, and vision, showing an adaptive evolutionary mechanism of convergent evolution." The paper's corresponding author said.
The high thermal conductivity of seawater makes it easier for animals to dissipate heat into the water, which is one of the main challenges faced by mammals returning to the sea from land. Marine mammals, on the other hand, return to the sea from land, but maintain the same surprising ability to control body temperature as they do on land. How do they do it?
The team found a clearer explanation for this question at the molecular level. "Different marine mammal branches have thermoregulation from the two aspects of thermogenesis and heat dissipation for thermoregulation," the corresponding author of the paper said, "both through the NFIA and UCP1 two sets of genes to regulate the synthesis and utilization of brown fat cells, so as to control the change of thermogenesis, and through the change of SMEA3E gene, so that the vascular system of marine mammals adaptive changes to regulate heat loss." Dual regulation ultimately achieves the maintenance of body temperature. ”
In fact, what the researchers found in the genomes of marine mammals is the potential use of these gene-coding instructions, namely that the upregulation or downregulation of the NFIA gene affects the cell fate of stromal precursors, the integrity of the UCP1 gene affects the normal functioning of brown fat cells, and the developed vascular system of marine mammals facilitates heat transfer to maintain constant body temperature.
Multiple genes undergo adaptive evolution
According to further introduction, in order to adapt to the aquatic environment, the skeletal form of marine mammals has also changed to varying degrees, such as forelimbs becoming flipper-like limbs, and the body is streamlined or spindle-shaped. The study found that in the biosynthetic pathway of proteoglycans, which is closely related to bone development, two key genes were specifically altered in marine mammals, which is very likely to affect changes in the morphology of marine mammals' bones.
In addition, the study also found that genes related to hypoxia tolerance, echolocation, deep diving and vision have undergone molecular-level adaptive evolution in marine mammals.
The research team has established a comprehensive marine mammal genome dataset, which provides more genetic evidence for the aquatic adaptation correlation traits of marine mammals and the convergence and evolution between different marine mammal branches, and provides good data support for researchers to carry out more in-depth research on the aquatic environment adaptation mechanism of marine mammals. At the same time, this study is of great significance for revealing the formation and maintenance mechanisms of global biodiversity, as well as exploring the relationship between environmental adaptation and species evolution.
However, the researchers believe that although some evidence has been found at the genetic level, to further reveal the molecular adaptation mechanism of marine mammals to the aquatic environment, it is necessary to obtain more high-quality whole genome data of species, as well as single-cell transcriptome data, proteome and metabolome data, and carry out a large number of more detailed comparative analysis and experimental verification.
"In addition, the molecular mechanisms of marine mammals' high INTELLIGENCE, high socialization, and longevity traits are still poorly understood, and further research is expected to make breakthroughs in these areas." The paper's corresponding author said.
Wang Zhuhua
Source: Science and Technology Daily