Abstract
Animals are generally capable of synthesizing eleven amino acids, while the remaining nine—often referred to as essential—must be acquired through diet. This metabolic trait profoundly shapes animal ecology and evolutionary trajectories, and recent phylogenomic studies indicate that it arose from gene losses at the root of the animal tree. However, it remains unclear which selective forces, if any, directed this far-reaching metabolic simplification. Here, we show that essential amino acids are energetically far more expensive to synthesize than non-essential ones, particularly under high respiratory conditions—a hallmark of the animal lifestyle. Using a probabilistic selection test, we find support for the hypothesis that energy-related selection, counterbalanced by pleio…
Abstract
Animals are generally capable of synthesizing eleven amino acids, while the remaining nine—often referred to as essential—must be acquired through diet. This metabolic trait profoundly shapes animal ecology and evolutionary trajectories, and recent phylogenomic studies indicate that it arose from gene losses at the root of the animal tree. However, it remains unclear which selective forces, if any, directed this far-reaching metabolic simplification. Here, we show that essential amino acids are energetically far more expensive to synthesize than non-essential ones, particularly under high respiratory conditions—a hallmark of the animal lifestyle. Using a probabilistic selection test, we find support for the hypothesis that energy-related selection, counterbalanced by pleiotropic effects, favored the outsourcing of essential amino acid production under well-oxygenated and nutrient-rich conditions. Remarkably, we also find that extant animals use expensive amino acids more frequently than their closest unicellular relatives, suggesting that constraints on their usage were relaxed once their synthesis was externalized, thereby enabling animal genes to evolve more freely across protein sequence space. In this light, we propose that the origin of animals was tightly linked to energy-related adaptations rather than to unpredictable stochastic events, as recently suggested.
Data availability
All data generated in this study are available in the supplementary materials and Figshare64 at: 10.6084/m9.figshare.30861554. Genome accession codes are available in the Supplementary Data 4 file. Source data are provided as a Source Data file. Source data are provided with this paper.
Code availability
The code used to perform the combinatorial phenotype selection test and auxotrophy detection is available in a GitHub repository65: https://github.com/nkasalo/MassiveOutsourcing.
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Acknowledgements
We thank M. Futo, A. Tušar, S. Koska, D. Franjević, and G. Klobučar for discussions. This work was supported by the Croatian Science Foundation under the project IP-2016-06- 5924 (T.D.-L.), the City of Zagreb (T.D.-L.), the Adris Foundation (T.D.-L.), the European Regional Development Fund, PK.1.1.10.0007 DATACROSS (M.D.-L. and T.D.-L.). We used the computational resources of the University Computing Center—SRCE (Padobran) and the Institute Ruđer Bošković. N.K. is enrolled in the doctoral program in Biology at the Department of Biology, Faculty of Science, University of Zagreb.
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Authors and Affiliations
Laboratory of Evolutionary Genetics, Division of Molecular Biology, Ruđer Bošković Institute, Zagreb, Croatia
Niko Kasalo & Tomislav Domazet-Lošo 1.
Department of Biology, Faculty of Science, University of Zagreb, Zagreb, Croatia
Niko Kasalo 1.
Department of Applied Computing, Faculty of Electrical Engineering and Computing, University of Zagreb, Zagreb, Croatia
Mirjana Domazet-Lošo 1.
School of Medicine, Catholic University of Croatia, Zagreb, Croatia
Tomislav Domazet-Lošo
Authors
- Niko Kasalo
- Mirjana Domazet-Lošo
- Tomislav Domazet-Lošo
Contributions
T.D.-L. and M.D.-L. initiated the study. N.K., M.D.-L., T.D.-L. conceptualized and performed the analyses. N.K. prepared the figures and tables for publication. N.K., M.D.-L., and T.D.-L. wrote the manuscript.
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Correspondence to Mirjana Domazet-Lošo or Tomislav Domazet-Lošo.
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Kasalo, N., Domazet-Lošo, M. & Domazet-Lošo, T. Outsourcing of energetically costly amino acids at the origin of animals. Nat Commun (2026). https://doi.org/10.1038/s41467-026-68724-6
Received: 29 November 2024
Accepted: 09 January 2026
Published: 22 January 2026
DOI: https://doi.org/10.1038/s41467-026-68724-6