Molecular fossils, also known as biomarkers, are organic molecules from ancient life preserved in natural archives like rocks, sediments, and crude oil. They are the remnants of biomolecules from organisms like plants and microbes. Unlike body fossils such as bones or shells, molecular fossils provide a record of life where physical remains are absent. These resilient chemical fingerprints carry information about ancient life and environments across millions of years.
How Molecules Become Ancient Records
The transformation of a biological molecule into a durable geological record is a selective process. It begins with biomolecules from living organisms, like the lipids that form cell membranes. When an organism dies, most of its organic material is broken down, but some molecules are more resistant to decay. Lipids are particularly resilient because they are hydrophobic.
These resilient molecules become incorporated into sediments, where they undergo chemical alterations known as diagenesis. Burial subjects these molecules to increasing heat and pressure over geologic time, transforming them into stable structures called geomolecules. A classic example is the conversion of cholesterol into the stable hydrocarbon cholestane.
The preservation of these molecular signatures depends on specific environmental conditions. Anoxic, or oxygen-poor, settings are effective because they limit microbial degradation. Rapid burial in sediment also helps to shield the molecules from decomposition, allowing them to persist while retaining a core structure linked to the original life form.
The Library of Life’s Chemical Traces
The variety of molecular fossils provides a rich library of information, as these biomarkers can be traced back to the organisms that made them. The major classes are hydrocarbons and derivatives of biological pigments.
Hydrocarbons are a major group of molecular fossils and include:
- Steranes, which are derived from sterols like cholesterol and serve as indicators of ancient eukaryotes, such as algae.
- Hopanes, which originate from bacteriohopanepolyols in the cell membranes of bacteria. The presence of specific hopanes can point to particular types of bacteria like cyanobacteria.
- N-alkanes, which are straight-chain hydrocarbons often derived from the waxes on plant leaves.
Derivatives of pigments form another category. Porphyrins are a notable example, derived from chlorophylls used in photosynthesis and hemes found in blood. Their discovery in petroleum by Alfred Treibs in 1936 was key evidence confirming the biological origin of crude oil. Carotenoid derivatives also leave a distinct trail, helping to identify the past presence of photosynthetic algae and bacteria.
Decoding Messages from Deep Time
Molecular fossils allow scientists to interpret Earth’s deep past by revealing the characteristics of ancient environments. The chemical structures of these molecules act as messages from ancient ecosystems, providing evidence for ancient bacterial communities through specific hopanes, for example.
Some biomarkers act as paleothermometers, allowing scientists to estimate past temperatures. The U-k’37 index, based on long-chain ketones from certain algae, is used to calculate past sea surface temperatures. Another method, the TEX-86 paleothermometer, uses lipids from aquatic archaea to achieve a similar goal.
Beyond temperature, biomarkers can illuminate the chemistry of ancient water bodies. For example, derivatives of a carotenoid called isorenieratane are produced by green sulfur bacteria, which require both light and anoxic, sulfur-rich water. Finding isorenieratane derivatives in ancient sediments thus points to a water column that was stratified, with an oxygen-free zone that reached into the sunlit depths. These clues help in understanding ancient nutrient cycles.
Unearthing Molecular Clues: Techniques and Methods
The study of molecular fossils involves a precise laboratory process to isolate and identify trace organic compounds. The process begins with careful sample collection to avoid modern contamination. In the lab, rocks or sediments are crushed, and solvents are used to extract the soluble organic matter containing the molecular fossils.
This complex mixture of molecules is then separated into its individual components using chromatography. In gas chromatography (GC), the mixture is vaporized and passed through a column that separates compounds based on properties like their boiling points. Liquid chromatography (LC) works on a similar principle but separates compounds in a liquid solvent.
Once separated, the individual molecules are identified using mass spectrometry (MS). Often coupled with chromatography (GC-MS or LC-MS), a mass spectrometer bombards molecules with electrons, breaking them into charged fragments. It then measures the mass-to-charge ratio of these fragments, creating a unique chemical fingerprint that allows for precise identification of the molecule’s structure.
Transformative Discoveries and Uses of Molecular Fossils
Research on molecular fossils has led to significant breakthroughs in our understanding of Earth’s history and has practical applications.
One of the most impactful discoveries was finding steranes in 2.7-billion-year-old rocks. This suggested that complex eukaryotic life may have existed long before the first undisputed body fossils appeared, sparking ongoing scientific investigation and debate.
In petroleum geology, molecular fossils are a standard tool. The analysis of biomarkers in crude oil helps geologists identify the source rocks from which the petroleum formed. By correlating molecular signatures in an oil sample to potential source rocks, companies can trace the origin and migration of hydrocarbons.
The insights from molecular fossils are also applied to paleoclimatology to reconstruct past climate changes. For instance, biomarker data has been used to detail rapid warming events in Earth’s history, such as the Paleocene-Eocene Thermal Maximum about 56 million years ago.
Knowledge of terrestrial biomarkers also guides the search for life on other worlds. Astrobiologists use this framework to identify potential molecular biosignatures that could be sought in samples from Mars or other celestial bodies.