Every summer starting around early May it seems that everywhere I look in my backyard a new anthill has popped up. Throughout town and when taking hikes at a local nature preserve it seems that I cannot take a single step without almost stomping on an anthill or running into some kind of bee or wasp. One thing that these common insects have in common is that they are social and live in large colonies together. This social lifestyle, called eusociality, has been remarkably successful with ants making up over 20% of the terrestrial animal biomass. A major question I want to look into is where did this social lifestyle come from?
Eusocial insects are insects that live in a colony that meets four criteria, adult individuals live in groups, cooperative care of juveniles, reproductive division of labor, and overlapping generations. The majority of eusocial insects are in the order Hymenoptera, home to bees, ants, and wasps, with the order Blattodea containing termites which is another major group of eusocial insects.
Figure 1: A) Bees B) Wasps, C) Ants, D) Termites
The order Hymenoptera is thought to have diversified between the Carboniferous and Triassic periods with the first known fossils dating to approximately 224 million years ago. The order Blattodea is thought to have originated in the Carboniferous around 350-300 million years ago with termites evolving from a primitive wood-eating roach during the Triassic between 200-250 million years ago.
Figure 2: Triassic Hymenoptera Magnicapitixyela dilettae
Despite the orders of the modern eusocial insects appearing in the Triassic it would take until the late Jurassic, roughly 100 million years later, for eusociality to evolve. It first appeared in termites with the earliest example of morphologically different castes being found in amber from Myanmar dated to 100-110 millions years ago in the late Jurassic. The first eusocial ants evolved in the early Cretaceous with examples of different castes found in New Jersey amber. Bees and wasps evolved complex social structures later with the earliest fossil evidence dating to the late Cretaceous, approximately 75 millions years ago.
Figure 3: Early Cretaceous amber showing two different species of ants in combat indicating the social behavior seen in modern ants of ant warfare
Current ideas on what drove the evolution of complex social behaviors among insect include ecological pressures, kin selection, and mutli-level selection. one example of an ecological path to the development of eusociality is thought to be solitary females rearing larvae in close proximity to protect from predators or use a common resource with the insects becoming less solitary over many generations due to selective pressures favoring a more social lifestyle. Kin selection indirectly increases fitness by promoting genes shared by relatives to carry on to future generations eusociality can be selected for over solitary behavior when the indirect fitness of helping a relative is higher than direct fitness for the individual. Mutli-level selection is another driving factor for the development. It posits that natural selection does not occur only at the individual level, but at the group level as well. As a result of group pressure genes that create cooperative groups can outcompete genes that lead to more individualistic individuals. Over time this can push a group to forming a eusocial colony society where the group is selected as if it is one superorganism where group fitness is more strongly selected for than individual fitness.
Social insects have become a highly successful since their evolution in the late Jurassic to early Cretaceous and can be found on every continent and in nearly every ecosystem type. Mechanisms such as ecological pressures, kin selection, and multi-level selection may have driven their evolution by increasing the importance of group fitness or individual fitness
References
https://www.nature.com/scitable/knowledge/library/an-introduction-to-eusociality-15788128/
https://www.pnas.org/doi/10.1073/pnas.011513798#:~:text=On%20average%2C%20ants%20monopolize%2015,not%20always%20run%20the%20world.
https://sasn.rutgers.edu/news/biologists-unravel-evolutionary-history-termites-and-cockroaches#:~:text=Their%20DNA%2Dbased%20analysis%2C%20combined%20with%20one%20of,much%20younger%20than%20scientists%20had%20previously%20thought.&text=But%20the%20team's%20paper%20shows%20they%20originated,million%20years%20ago%2C%20in%20the%20Cretaceous%20period.
https://www.nature.com/articles/s42003-024-06678-5/figures/3
https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2021.732907/full
https://entomologytoday.org/2016/02/12/ant-and-termite-fossils-indicate-advanced-sociality-100-million-years-ago/
https://www.sciencedirect.com/science/article/pii/S0960982216000889
https://www.annualreviews.org/content/journals/10.1146/annurev.ecolsys.28.1.27?utm
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