Adaptive loss of function accelerated the evolution of ancient and modern human cognition

By introducing a new method called FASTER, the researchers demonstrate that human evolution has been significantly driven by the accelerated loss of molecular functions—such as reduced protein stability and decreased regulatory activity—particularly in genomic regions related to brain development and cognition.

The Gist

For a long time, scientists have looked for signs of rapid evolution in the human genome. By comparing our DNA to that of chimpanzees, our closest living relatives, researchers have found many specific regions where the genetic code changed much faster in humans than in other primates. However, most previous methods focused only on changes in the sequence of the DNA itself—the specific order of the chemical letters that make up our genetic instructions. This left a major question unanswered: did the actual biological functions controlled by these regions change rapidly, or was it just the sequence of letters that changed?

In this paper, the researchers introduce a new method called FASTER (Function Aware Statistical Test for Evolutionary Rates). Unlike older tools, FASTER can detect whether the predicted biological function of a genomic region is changing at an accelerated rate. This allows the researchers to look beyond just the sequence of letters and examine how the work those letters do within a cell is evolving.

By applying FASTER to humans and chimpanzees, the researchers identified many different types of genomic regions—including those that code for proteins, those that sit at the ends of genetic instructions, and those that do not code for proteins at all—that show an accelerated evolution of function. A consistent pattern emerged: in the human lineage, these functional changes occurred much more frequently in sites that were previously highly conserved, meaning they had remained unchanged for a long time.

The researchers found that many of these changes are predicted to reduce the stability of proteins or decrease the accessibility of chromatin, which is the structure that holds DNA inside a cell. These changes often affect how genes are turned on or off.

Multiple lines of evidence suggest that this rapid change in human function was driven by positive selection related to brain development and cognition. The paper also suggests that this evolutionary process has continued to shape human biology even within the last several thousand years. Collectively, the results demonstrate that a widespread, accelerated reduction in biological function—specifically in the activity that regulates genes—may have been a major driver of both ancient and recent human evolution.

Technical Summary

Technical Summary: Adaptive Loss of Function Accelerated the Evolution of Ancient and Modern Human Cognition

1. Problem Statement

Current methodologies for detecting accelerated evolution in the human lineage primarily focus on sequence-level acceleration within short, non-coding regions (e.g., Human Accelerated Regions or HARs). While these methods identify genomic areas that change more rapidly in humans than in chimpanzees, they suffer from two major limitations:

• Sequence vs. Function: They detect changes in the nucleotide sequence itself but cannot distinguish whether those changes actually alter the biological function of the genomic element.
• Scope of Application: Existing methods are often restricted to specific types of non-coding elements, leaving it unclear whether accelerated evolution is a genome-wide phenomenon affecting protein-coding genes, untranslated regions (UTRs), and diverse regulatory elements.

2. Methodology: The FASTER Approach

To address these gaps, the authors developed FASTER (Function Aware Statistical Test for Evolutionary Rates).

• Core Innovation: Unlike sequence-centric models, FASTER evaluates the rate of change in predicted function. It integrates genomic sequence data with functional annotations (such as protein stability scores or chromatin accessibility predictions).
• Mechanism: The method compares the rate at which functional properties change across different lineages. If a specific functional trait (e.g., the ability of a DNA sequence to bind a transcription factor) decays or shifts more rapidly in humans than in chimpanzees, FASTER flags this as accelerated functional evolution.
• Versatility: The framework is agnostic to the type of genomic region, allowing it to be applied to protein-coding sequences, UTRs, and various non-coding regulatory elements simultaneously.

3. Key Contributions

• Development of FASTER: A novel computational framework that shifts the paradigm from detecting "fast-changing DNA" to "fast-changing biological function."
• Genome-wide Functional Mapping: The study provides a comprehensive map of functional acceleration across multiple layers of the genome (coding, UTR, and non-coding).
• Identification of "Adaptive Loss": The study introduces the concept that evolution does not always act by gaining new functions, but often by the accelerated loss or reduction of existing functions (e.g., reducing protein stability or decreasing regulatory activity).

4. Results

• Widespread Functional Acceleration: The authors identified significant acceleration of function across protein-coding regions, UTRs, and non-coding elements.
• Directionality of Change: A consistent pattern emerged: human evolution is characterized by an accelerated reduction in function at highly conserved sites. Specifically, many human-accelerated sites are predicted to:
  • Decrease protein stability.
  • Reduce chromatin accessibility (cis-regulatory activity).
• Biological Enrichment: The regions showing accelerated functional loss are significantly enriched for genes involved in brain development and cognition.
• Temporal Continuity: The evidence suggests that this process is not merely an ancient phenomenon; the acceleration of functional loss has continued to shape the human genome even within the last several thousand years.

5. Significance

This paper fundamentally alters our understanding of human evolutionary drivers. It suggests that the "uniqueness" of the human genome may not stem from the sudden appearance of complex new functions, but rather from the systematic and rapid tuning down of ancestral functions.

By demonstrating that the accelerated reduction of cis-regulatory activity and protein stability is a major evolutionary force, the study provides a new lens through which to view human cognitive evolution. It moves the field toward a more nuanced "functional" view of genomics, where the loss of ancestral constraints is seen as a primary mechanism for driving phenotypic innovation in the human brain.