Which step from chemistry to life remains hardest for scientists to explain?

Why making life's building blocks is no longer the main puzzle

For decades, scientists focused on how the molecular ingredients of life — amino acids, sugars, fatty acids, nucleotides — could form under prebiotic conditions. That part of the problem is largely solved. A 2023 review notes that there are now 'a handful of prebiotically plausible scenarios that enable the laboratory synthesis of most amino acids, fatty acids, simple sugars, nucleotides and core metabolites' [1]. Even phosphate, long considered a scarce and limiting nutrient, appears to have been abundant: experiments and models show that in the iron-rich seawater of the early Earth, phosphate concentrations were roughly 1,000 to 10,000 times higher than previously estimated, easily meeting the needs of primitive cells [2]. So the raw materials were there.

The real difficulty is not supply — it's organization. As the same review puts it, 'the major bottleneck then seems to be the self-organization of those building blocks into systems that can self-sustain' [1]. This is a fundamentally different kind of problem: not a chemical synthesis puzzle, but a systems chemistry puzzle about how molecules cooperate, compartmentalize, and maintain themselves far from equilibrium.

The real hurdle: getting molecules to self-organize into a living system

The gap between a mixture of biomolecules and a living cell is enormous. A 2026 analysis using information theory and algorithmic complexity estimates the 'formidable entropic and informational barriers' to assembling a viable protocell within the available window of Earth's early history [5]. In plain terms, the odds of the right molecules coming together in the right arrangement by random chance alone are astronomically low — the paper calls it 'the unreasonable likelihood of being' [5]. This is not about whether the molecules exist, but whether they can be wired into a network that is self-sustaining, autocatalytic, and capable of replication.

Several papers converge on the same conclusion from different angles. One proposes that life emerged as 'a novel property of a prebiotically assembled system — formed through the integration of distinct molecular worlds' that interact via catalytic, autocatalytic, and self-assembly processes [4]. Another suggests that mineral surfaces like hydroxyapatite could have helped by concentrating molecules and catalyzing reactions, but even then, the challenge of sustaining 'chemical reactivity and promoting increasing levels of molecular complexity' under dynamic, non-equilibrium conditions remains critical [3]. A third paper argues that the missing link may be a small ancestral RNA that acted as a 'proto-ribosome' to synthesize the first peptides, bridging the gap between RNA and protein worlds [7]. All of these point to the same bottleneck: getting molecules to cooperate in a sustained, organized way.

What a successful protocell would require — and why it's so hard

A self-sustaining protocell needs three things that are extremely difficult to achieve simultaneously: a boundary (like a membrane) to separate inside from outside, a metabolism to extract energy and build new molecules, and a genetic system to store and replicate information. These components must work together in a system that is 'out of equilibrium' — constantly consuming energy from the environment to maintain its internal order [1]. A 2023 paper emphasizes that the key is 'how recursively changing conditions could help them engage in self-organized and dissipative networks/assemblies' [1]. This is not a one-time assembly; it is a dynamic, ongoing process.

The difficulty is compounded by the fact that early Earth conditions were not uniform. One hypothesis points to volcanic island lightning as a focal point for generating prebiotic ingredients [6], while another highlights the role of iron-rich seawater in making phosphate available [2]. But even with the right ingredients and energy sources, the step from chemistry to the first cell remains unexplained. As one paper puts it, 'uncovering physical principles for life's spontaneous emergence remains a grand challenge for biological physics' [5]. The field has moved from asking 'what molecules?' to asking 'how do they organize?' — and that is the question that still lacks a clear answer.

About These Sources

This answer is built on 7 peer-reviewed studies — published from 2022 to 2026, 3 from 2024 or later, 5 in Q1 journals, collectively cited 115 times — selected as the most relevant from 8 studies that passed quality screening, drawn from 62 papers retrieved from a database of over 500 million.

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