How much of the fossil record is still missing?

Systematic biologists cultivate an exceptional garden. Following in the footsteps of Charles Darwin, they construct phylogenetic trees – branching charts that trace evolutionary descent to a common ancestor. Also called phylogenies, these diagrams and classifications map biodiversity at various levels, from individual organisms to larger taxonomic groups such as kingdoms and domains.

With each tree, these biologists move closer to revealing something even more profound: a four-dimensional representation of life itself. The fossil record is key to building this tree of life, but how does it function? And how far are we from completing the record?

Reconstructing Earth's Past from Fossilized Remains

To fill in these missing gaps, scientists utilize several techniques. One major tool is cladistics, a method used to hypothesize the evolutionary relationships among organisms. It’s similar to building a family tree with some ancestors still unknown. Scientists also rely on molecular sequencing to reveal the hierarchy of relationships between species by comparing their molecular details.

Imagine creating a family tree once again, but this time using DNA evidence to determine where each organism belongs on the chart. This is the fossil record: the preserved remnants of ancient life forms, trapped within the Earth. By mapping the locations of fossils within rock layers, scientists can deduce when certain organisms lived in geological time.

Fossilized Evidence of Life from the Past

The fossil record is, however, far from complete. One significant reason for this is that sediment must cover organic material for fossilization to begin. Unfortunately, most organisms decompose before this can occur.

The chances of fossilization increase if the organism existed in large numbers or lived in environments near sedimentary rocks. For example, trilobites, ancient marine arthropods, fit both criteria and are commonly found as fossils. In contrast, the Tyrannosaurus rex is much rarer. It was large, land-dwelling, and a top predator, making it a smaller portion of the population.

Although fossils are commonly preserved in stone, they aren't immune to the elements. Like all rocks, they can erode, melt, and break apart. When you consider the fossils that remain undiscovered and those we cannot fully interpret (due to incomplete fossilization or limitations in technology), the fossil record becomes even more fragmented.

Much like the mineralized bones themselves, the fossil record is an imperfect structure that scientists enhance using various methods. While cladistics, molecular sequencing, and fossil evidence each provide different sets of data, systematic biologists often uncover similar patterns of diversification across all three. In other words, these methods work in tandem, offering a coherent view of what the tree of life should look like.

The Difficulty of Precisely Tracing Fossils

The further back we go in time, the more incomplete the fossil records become. Organisms from more recent times are also missing. For instance, freshwater mollusks of the class Bivalvia experience up to 45 percent incompletion in certain subclasses [source: Valentine et al.]. Additionally, important ancient animals that should connect major phyla remain elusive, such as the last common ancestors. Ongoing research into fossilization helps us understand just how much of the record is still missing.

Therefore, on its own, the fossil record is quite lacking in many areas. However, like fingerprints at a crime scene, it is just one part of the whole picture. Fossils, cladistics, and molecular sequencing work together to provide a larger, more accurate understanding of the evolution of life [source: Benton et al.].