Chapter 10: The Neural Dark Ages

Chapter Overview:

• Main Focus: This chapter explores a period of relative stasis in brain evolution, the "Neural Dark Ages," occurring between the emergence of the vertebrate brain template and the significant innovations of the mammalian brain. Bennett argues that while other aspects of vertebrate bodies underwent significant changes during this time, brain architecture remained relatively unchanged, with innovation instead focused on physical and mechanical adaptations like fins, gills, lungs, and legs (Bennett, 2023, p. 166).
• Objectives:
• Define the "Neural Dark Ages" and its temporal boundaries.
• Explain why brain evolution stalled during this period while evolutionary innovations in other biological mechanisms flourished.
• Describe the environmental pressures and evolutionary changes in other species that occurred during this time, particularly the transition of vertebrates onto land.
• Set the stage for the next major breakthrough in brain evolution – the emergence of the neocortex in mammals.
• Fit into Book's Structure: This chapter provides a crucial link between the establishment of the vertebrate brain template (Chapter 5) and the subsequent explosion of intelligence in mammals (Chapter 11). It emphasizes that evolution is not always a continuous process of improvement, but can involve periods of stasis punctuated by bursts of innovation. It also highlights how environmental changes can create both opportunities and challenges for different lineages.

Key Terms and Concepts:

• Neural Dark Ages: A period of relatively little change in vertebrate brain architecture, lasting from approximately 420 to 375 million years ago (the Devonian period) through the Permian period up to approximately 250 million years ago when mammals began to emerge (Bennett, 2023, p. 166, 168-169, 206). Relevance: This period contrasts with the rapid brain evolution seen in earlier and later periods.
• Devonian Period: A geologic period spanning from 419.2 to 358.9 million years ago. Relevance: This period saw the diversification of fish and the transition of some vertebrates onto land. It also saw the Late Devonian Extinction Event, which had a dramatic impact on species diversity in the oceans and was the bottleneck from which the next evolutionary stage of air-breathing tetrapods emerged (Bennett, 2023, p. 166-167).
• Permian-Triassic Extinction Event: A mass extinction event that occurred about 252 million years ago, wiping out a vast majority of species on Earth. Relevance: This catastrophic event created new ecological opportunities for the surviving lineages, including the ancestors of mammals. It was also the catalyst for a major shift in the ruling class of land animals, from therapsids to reptiles, which created the predatory conditions from which warm-bloodedness, miniaturization, and nighttime hunting strategies evolved in mammals (Bennett, 2023, p. 169).
• Tetrapods: Four-limbed vertebrates. Relevance: Tetrapods evolved from fish during the Devonian period, representing a major step in the transition to land.
• Amniotes: Vertebrates that lay eggs on land or retain the fertilized egg within the mother. Relevance: Amniotes, including reptiles, birds, and mammals, evolved adaptations that allowed them to reproduce without relying on water.
• Therapsids: A group of synapsids that includes the ancestors of mammals. Relevance: Therapsids were the dominant land animals during the Permian period, before being largely wiped out by the Permian-Triassic extinction event. The only surviving lineage of therapsids was the small cynodonts who survived by digging underground and who would emerge as the ancestors of the first mammals (Bennett, 2023, p. 169-170).
• Cynodonts: A group of therapsids that includes the direct ancestors of mammals. Relevance: Cynodonts evolved several mammalian traits, such as warm-bloodedness and specialized teeth. They would emerge to become the first mammals (Bennett, 2023, p. 170).

Key Figures:

• No specific scientists or researchers are mentioned by name in this chapter, as the focus is on broader evolutionary trends and ecological pressures.

Central Thesis and Supporting Arguments:

• Central Thesis: The "Neural Dark Ages" was a period of relative stasis in vertebrate brain evolution, during which innovation was focused on physical and physiological adaptations rather than neural architecture. This period was punctuated by significant extinction events and set the stage for the dramatic expansion of the neocortex in the brains of the first mammals.
• Supporting Arguments:
• Lack of significant brain changes: Fossil evidence suggests that the basic vertebrate brain template remained relatively unchanged during this period.
• Environmental pressures: The transition to land and the subsequent diversification of tetrapods and amniotes created selective pressures that favored adaptations in other biological systems, not the brain.
• Extinction events as catalysts: The Late Devonian and Permian-Triassic extinctions reshaped ecosystems and created opportunities for new lineages to thrive, including the ancestors of mammals. It was the evolutionary ‘bottleneck’ of the Permian-Triassic Extinction that created the evolutionary pressures to diversify reptiles, and for cynodonts to survive via warm-bloodedness, miniaturization, and nighttime foraging strategies (Bennett, 2023, p. 169-170).

Observations and Insights:

• Evolutionary stasis is not uncommon: Periods of relative stability punctuated by bursts of change are a common pattern in evolutionary history.
• Environmental change drives adaptation: The transition to land and the fluctuating climate of the Permian period led to significant adaptations in body size, temperature regulation, and reproductive strategies.

Unique Interpretations and Unconventional Ideas:

• Focus on the "Neural Dark Ages": Bennett’s emphasis on stasis in brain evolution contrasts with narratives that focus solely on progress and innovation. This concept of the ‘Neural Dark Ages’ frames brain evolution with a new lens—by highlighting the timing of when brain evolution occurred and when it did not occur, the author subtly suggests that there may be underlying mechanisms causing both stasis and change which are worthy of further investigation.

Problems and Solutions:

Categorical Items:

Bennett uses established biological classifications (fish, amphibians, reptiles, therapsids, cynodonts) to organize his discussion of vertebrate evolution. He also introduces the categories of life into three levels of complexity from one billion years ago: single-celled organisms, small multicellular life, and large multicellular life (Bennett, 2023, p. 25).

Literature and References:

• Scientific literature on the Devonian period, Permian-Triassic extinction event, and vertebrate evolution is cited.

Areas for Further Research:

• The factors contributing to the stasis in brain evolution during the "Neural Dark Ages" need further investigation.
• The specific adaptations that allowed vertebrates to thrive on land require more detailed study.
• The evolutionary relationship between therapsids, cynodonts, and mammals is an area of ongoing research.

Critical Analysis:

• Strengths: The chapter provides a valuable counterpoint to narratives of continuous progress in evolution, highlighting the importance of environmental context and ecological pressures.
• Weaknesses: The chapter could benefit from more detailed discussion of the specific brain structures and functions of the animals discussed. The author makes a subtle comparison between the evolution of complexity and intelligence in biological systems and climate change which, though perhaps meant to demonstrate how over-proliferation can create a sort of ecosystem “collapse,” is not entirely clear nor well-supported (Bennett, 2023, p. 158).

Practical Applications:

• Understanding the dynamics of evolutionary stasis and change can inform our understanding of long-term trends in technological and cultural evolution.

Connections to Other Chapters:

• Chapter 5 (Cambrian Explosion): This chapter follows Chapter 5 by exploring the subsequent period of vertebrate evolution on land.
• Chapter 11 (Generative Models): This chapter sets the stage for the emergence of the neocortex, which marks the end of the "Neural Dark Ages" and the beginning of a new era of rapid brain evolution, ending a period where evolutionary progress seemed to occur almost entirely in body plans and physical adaptations rather than in brains (Bennett, 2023, p. 166).

Surprising, Interesting, and Novel Ideas:

• The concept of the "Neural Dark Ages": This idea challenges the traditional narrative of continuous progress in brain evolution, highlighting a period of relative stasis. (Bennett, 2023, p. 157-158)
• The role of extinction events as catalysts for change: The Late Devonian and Permian-Triassic extinctions are presented as crucial events that reshaped ecosystems and created new opportunities for surviving lineages (Bennett, 2023, p. 158-160, 162, 169).
• The emergence of warm-bloodedness as a key adaptation: This physiological innovation allowed therapsids, the ancestors of mammals, to thrive in fluctuating environments and ultimately survive the Permian-Triassic extinction event (Bennett, 2023, p. 160).

Discussion Questions:

• What factors might contribute to periods of stasis in evolution, and how can we identify such periods in the fossil record?
• How did the transition to land create new challenges and opportunities for vertebrate evolution?
• What were the long-term consequences of the Permian-Triassic extinction event, and how did it shape the evolution of mammals?
• How does Bennett's concept of the "Neural Dark Ages" challenge our understanding of the pace and direction of evolutionary change?
• What can the “Neural Dark Ages” tell us about the conditions which give rise to innovation in biology or even in technology?

Visual Representation:

[Vertebrate Brain Template (Chapter 5)] --> [Neural Dark Ages (Stasis in Brain Evolution, Environmental Change, Extinction Events)] --> [Emergence of Mammals (Neocortex)]

TL;DR