LUCA’s Complexity Challenges Evolutionary Origin of Life

This past Christmas was a lot of fun for Grammy and Poppy as we watched two of our grandkids unwrap their presents. It wasn’t much fun trying to assemble all their cool new toys, though. The pressure was on. They wanted to play with their new toys . . . and they wanted to play with them now! 

Unfortunately, the toy designs turned out to be much more elaborate and complex than we imagined when we purchased them. And the assembly directions weren’t helpful. The instructions gave the impression that the assembly process involved a handful of easy steps. Oh, how misleading. 

Discovering LUCA’s Complexity
In the same way we struggled to assemble Christmas toys, evolutionary biologists have struggled to piece together a clear picture of life in one of its earliest forms—LUCA (life’s universal common ancestor). It now appears that LUCA was far more elaborate and complex than most evolutionary biologists had imagined. This conclusion comes from a team of collaborators from the US and the UK.1

Estimating LUCA’s complexity is no easy task. Previous studies that relied on DNA—life’s instruction manual—were misleading and underestimated LUCA’s intricacy. Using a completely different approach to characterize LUCA—one that focused on its physiological features instead of the gene set encoded in its DNA—the international research team discovered that LUCA was perhaps more complex than contemporary bacteria and archaea. 

In the abstract of their paper, the researchers write: 

“Our results depict LUCA as likely to be a far more complex cell than has previously been proposed, challenging the evolutionary model of increased complexity through time in prokaryotes. Given current estimates for the emergence of LUCA we suggest that early life very rapidly evolved considerable cellular complexity.”2 

This insight has far-reaching scientific implications and challenges the mainstream perspective about the mode and tempo for the origin and early evolution of life. It also has implications for the design argument. And it confirms key predictions of RTB’s model for life’s origin. 

Before I describe this study and elaborate on its implications for various origin-of-life models, a brief bit of background might be helpful for readers unfamiliar with LUCA.  

The Last Universal Common Ancestor (LUCA)
The concept of LUCA is endemic to the evolutionary paradigm. In this framework, LUCA would have been a population of single cells at the root of the evolutionary tree of life.

Evolutionary biologists don’t regard LUCA as the first organism on Earth. They believe that cells existed prior to LUCA. They refer to these cells as progenotes. LUCA, however, represents the organism that evolutionary biologists think was the common ancestor that gave rise to all existing life-forms. Accordingly, LUCA produced the two major divisions of the evolutionary tree, Bacteria and Archaea. 

Figure 1: LUCA and the Tree of Life 
Credit: Wikipedia

At one time, evolutionary biologists entertained the possibility that LUCA was a collection of different organisms interconnected through horizontal gene transfer. More recently, life scientists have adopted the view that LUCA consisted of a single cell type.3 Yet, evolutionary biologists think that LUCA coexisted with a collection of other distinct cell types. LUCA was simply the cell type “lucky” enough to give rise to evolutionary lineages that persisted throughout life’s history. 

Determining LUCA’s Features
One of the abiding interests of evolutionary biologists is to identify LUCA’s characteristic features. This project has been shaped by two points of conviction. The first is that LUCA was a relatively simple, primitive cell. The second is the notion that cellular complexity would accrue gradually over a long time. Both convictions are influenced by evolutionary gradualism, where life progresses in a stepwise manner from simple life-forms to more complex. 

Support for this view seemingly comes from comparative genomics studies in which evolutionary biologists seek to identify LUCA’s gene set from the genes that are common to all bacterial and archaeal organisms. If LUCA gave rise to the evolutionary lineages that produced bacteria and archaea, then the common set of genes shared by these two groups of microorganisms must have contributed to LUCA’s genome. Estimates from these types of studies place the gene set for LUCA somewhere between about 350 to 1000 genes, depending on the study design. 

Other studies use model organisms to pursue the LUCA gene set. Operating with the presumption that LUCA must have been a simple, primitive cell, the model organisms selected are microorganisms with near-minimal genomes, such as parasites, endosymbionts, and microbes with streamlined genomes. 

These two approaches have technical limitations. (I don’t have the space to discuss them here.) They also don’t provide an adequate picture of LUCA’s morphology (shape, form) and molecular physiology. Part of the challenge arises from our limited understanding of how genotype (DNA) translates into phenotype (physical and behavioral traits)—even at the cellular level. 

New Insights into LUCA’s Complexity
To address these concerns (and others), the research team adopted a different approach by which they attempted to reconstruct LUCA’s cellular traits from evolutionary trees built using 28 traits distributed among 3,128 bacterial and archaeal species. 

From this analysis, they recovered 22 physiological and morphological features for LUCA. According to their findings, LUCA:

  • Was actively motile (capable of movement)
  • Had a single-cell wall
  • Possessed a single-cell membrane 
  • Contained a mixture of bacterial and archaeal lipids in the cell membrane 
  • Was tolerant to salt water with less salinity than modern-day seawater
  • Lived in high-temperature environments with temperatures above 70° C 
  • Lived in waters with neutral pH values
  • Lived in an environment devoid of oxygen
  • Used inorganic materials as an oxidation source
  • Was ovoid in shape

In technical terms, LUCA was a halotolerant, hyperthermophilic, chemolithoautotrophic, anaerobe. In lay terms, LUCA appears to have been surprisingly complex. 

Using morphological and physiological features to estimate LUCA’s features reveals complexity that wasn’t evident from the instructions found in the reconstructed gene sets. In fact, the genetic complexity required to support LUCA’s morphological and physiological complexity far outpaces the estimated complexity from reconstructing LUCA based on comparative gene studies. In fact, the research team discovered that LUCA possessed a genome that was about 2.49 million base pairs in size (similar in size and complexity to a typical bacterial genome). 

When Did LUCA Live?
One way to address this question about the timing of LUCA’s appearance is by studying the geological, geochemical, and fossil data. A preponderance of evidence indicates that cellular life unequivocally appears on Earth as early as 3.8 billion years ago. There is some evidence, though controversial, that life may have first appeared on Earth between 4.2 and 4.4 billion years ago. 

The fossil and geochemical evidence doesn’t tell researchers much about the characteristics of these early cells beyond size, overall shape, and some insight into their metabolic lifestyle. The evidence doesn’t pinpoint when LUCA existed. 

A research team from the University of Bristol (UK) sought to clarify this ambiguity by using a molecular clock analysis to locate key events in life’s early history.4 They conclude that eukaryotic cells first appeared around 1.8 billion years ago, bacteria and archaea appeared around 3.4 billion years ago, and LUCA showed up around 3.9 billion years ago. 

These findings cause consternation for gradualistic evolutionary models when Earth’s early planetary history is considered. Earth forms at 4.5 billion years ago. For the first few hundred million years, Earth must have been a molten planet. At the time, no crust would have existed on Earth and the water on the planet’s surface would have been steam in the atmosphere. 

Evidence indicates that by about 4.2 billion years ago, Earth cooled sufficiently for the formation of the crust and oceans to occur. Around 3.8 billion years ago, however, Earth’s relative calm was interrupted by the Late Heavy Bombardment (LHB), during which comets and asteroids pummeled the inner solar system planets. Controversy exists about the magnitude of the LHB. Some models have Earth returning to a magma world with oceans completely volatilized, once again becoming steam. In this scenario, any life on Earth would have been exterminated. 

Other models view the LHB as less catastrophic and suggest that only life on Earth’s surface was lost. Microbes deep in Earth’s crust likely survived. Still other models indicate that the severity of the LHB was limited. In these models, surface and subsurface life persisted through the LHB. 

It is remarkable to think that LUCA originated near the time of the LHB. At one extreme, if the LHB was severe, and LUCA either appeared before the LHB (and was subsequently exterminated) or right after the LHB, it leaves little time for chemical evolution to produce progenotes that, in turn, evolved the remarkable complexity displayed by LUCA. At the other extreme, if LUCA originated before the LHB and survived the series of impactors striking Earth, it still leaves little time for life to originate and evolve the complexity that characterizes LUCA. In the former scheme, the origin and evolution of life occurred over tens of millions of years. In the latter scenario, the most likely window of time for life’s origin and the evolution of LUCA is around 200 million years. Neither timescale fits mainstream thinking among evolutionary biologists. 

As the research team writes:

“Our results have the potential to push cellular complexity back to the very beginning of life. Barring the unlikelihood of panspermia, these results imply that complex phenotypic traits arose far earlier in the history of life than previously thought. . . . early life may have very quickly evolved considerable cellular complexity. We thus reveal LUCA as a potentially complex cell possessing a genetic code perhaps more intricate than many modern bacteria and archaea.”5

As I pointed out, these possibilities leave little time for a gradualistic evolution of the earliest cellular life. They also raise questions about how life could have evolved that level of cellular complexity so rapidly. 

The RTB Creation Model for Life’s Origin
While the latest insight into the complexity of LUCA fits uncomfortably within an evolutionary framework, it finds a place in our creation model. In Origins of Life, Hugh Ross and I present a biblically based scientific model for life’s origin.

Our model finds inspiration in the creation accounts found in the biblical text. It also adopts the perspective that a Creator played a direct role in the origin and history of life. 

One key feature of our model, if it’s valid, is that it makes predictions about what science should discover. (Details of our model and the ensuing predictions can be found in Origins of Life.)

Some of the germane predictions of our model regarding the complexity of the first life-forms include:

  1. Life should appear early in Earth’s history. 
  2. Life should originate under hostile conditions.
  3. Life should originate rapidly.
  4. First life should be complex.

The latest scientific findings about LUCA comport nicely with the RTB model for life’s origin. They also satisfy these predictions. In doing so, these advances give scientific credibility to the creation passages that pertain to early Earth and the origin of life. 

In short, the sudden, rapid, and early appearance of complex life is precisely the signature we would expect of a Creator and is indeed a response for the origin and early history of life. 

What could be simpler than that?


RTB’s Creation Model for the Origin of Life

The Timing of First Life on Earth

The Late Heavy Bombardment

Life’s Minimum Complexity


  1. Fouad El Baidouri et al., “Phenotypic Reconstruction of the Last Universal Common Ancestor Reveals a Complex Cell,” bioRxiv (October 10, 2021), doi:10.1101/2020.08.20.260398.
  2. Baidouri et al., “Phenotypic Reconstruction.”
  3. Douglas L. Theobald, “A Formal Test of the Theory of Universal Common Ancestry,” Nature 465 (May 13, 2010): 219–222, doi:10.1038/nature09014
  4. Holly C. Betts et al., “Integrated Genomic and Fossil Evidence Illuminates Life’s Early Evolution and Eukaryote Origin,” Nature Ecology and Evolution 2 (October 2018): 1556–1562, doi:10.1038/s41559-018-0644-x
  5. Baidouri et al., “Phenotypic Reconstruction.”