Ancestral brains of Arthropods revealed by new fossils

Sneha Singhal, Jaypee Institute of information technology, Noida

Arthropods and their fossils

Among the most diverse and species-rich groups of animals are arthropods, which include crustaceans, insects, scorpions, spiders, millipedes, and centipedes, just to name a few. Researchers have long hypothesized that the archetypal brain shared by all arthropods comes from fossils preserved to extremely high detail by creatures from over half a billion years ago. The fossils, which belong to the arthropod Leanchoilia, confirmed, as predicted by earlier genetic studies of insect and spider embryos, that there is an abscissa of the brain in the extreme frontal region that is inaccessible in modern arthropods. Even though this part of the arthropod’s brain is invisible, it is composed of several crucial neural centers, including stem cells that are ultimately responsible for decision-making and memory functions. Prosocerebrum was named for its ability to be distinct from forebrain, midbrain, and hindbrain present in living arthropods; Proso means “front.” 

Nicholas Strausfeld, lead author of a recent paper published in the journal Current Biology, states that the fossils described in his paper are unlike anything seen before. The two preserved nervous systems indicate that this most anterior part of the brain was present and structurally distinct half a billion years ago, before the three segments that make up the forebrain, midbrain, and hindbrain evolved. Some of the findings of the paper will be found in the following sections.

It is rare to find fossilized brains

Fossilized remains of Leanchoilia have been found in rock strata of the Kaili formation in Guizhou province, southwest China. They date back 508 million years to the Cambrian period. Iron-rich sedimentary rock hosts the Kaili fossils, which preserved soft tissues, which later became coated with carbon deposits due to the presence of iron. Recent developments reveal that the Kaili formation fossils of early stem arthropods preserve neural tissue that can provide a glimpse into the primitive brain of an animal that existed when animal life began. 

Soft tissues like nervous systems are difficult to fossilize. Thus, it poses a challenge to study the development of neural systems in the early evolutionary stage. In addition to providing new evidence for the evolutionary origins of two different visual systems in arthropod evolution, the fossils also shed light on the genetic relationship between pairs of front-facing eyes and sideways-looking eyes, which share descendants with modern-day species. 

Nauplius eyes, or ocelli, resemble spider and scorpion eyes structurally. These simpler eyes are found in living arthropod embryos and are closely related to those of a protocerebrum’s forward eyes in Leanchoilia. A similar pattern appears in Leanchoilia, where the sideward eyes correspond to the protocerebrum, which is the segmental ganglion behind the protocerebrum that defines the arthropod forebrain. In arthropods, the protocerebrum provides the compound eyes of crustaceans, and insects or lateral-lens eyes for arachnids, centipedes, and millipedes. A portion of the protocerebral region is also responsible for serving those eyes’ visual centers. As such, these fossils demonstrate that the protocerebrum of modern arthropods contributed to the development of ganglia as well as frontward eyes. As these fossils confirm, the labrum still evolved from the grasping appendages of Radiodonta, stem arthropods that were large predators in the Cambrian period. 

Effects of brain evolution on vertebrates

As well as filling a century-old knowledge gap regarding the evolution of arthropod brains, Strausfeld says the findings have implications for the emergence of vertebrate brains. Even though simple, fish-like arthropods existed along with these fossilized arthropods, no fossil records exist for any of their brains, making no evidence for protocerebrum invertebrates. Recent studies have shown that genes defining mice’s forebrain, midbrain, and hindbrain correspond to genes defining arthropod brains’ three ganglionic divisions. The higher centers in the arthropod brain, which originate in the ancient arthropod protocerebrum, have some genetic correspondences with certain crucial centers involved in decision making, learning, and memory invertebrates. 

So, the common ancestor of both vertebrates and invertebrates possessed the basic circuits for basic cognition and decision making even before the Cambrian period, possibly even before the evolution of segmentally organized body plans. However, no fossils have been found indicating a discrete, nonsegmental domain in these early vertebrate ancestors. It is reasonable to hypothesize, however, that vertebrates may possess ancient, non-segmented parts of their brains that are not detectable in arthropods until recently, such as Leanchoilia.

Also read: Cathepsins: Lysosomal Proteases that are Vital and Lethal

Reference:

• Stolte, D., & Arizona, U. of. (n.d.). New fossils show what the ancestral brains of arthropods looked like. Retrieved August 21, 2021, from https://phys.org/news/2021-08-fossils-ancestral-brains-arthropods.html

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