Rocks vs Clocks: When Did Modern Birds Really Appear?

Fossil data suggest that birds diverged from dinosaurs in the Cenozoic, whereas molecular clock data indicate the avian lineage is much older than that

by GrrlScientist for ScienceBlogs | @GrrlScientist

The first feathered dinosaur fossil found in China — Sinosauropteryx.
The feathers can be seen in the dark line running along the specimen’s back. (Credit: Mick Ellison, AMNH [larger view])

There is a lot of controversy among scientists regarding when modern birds first appeared. The current fossil record suggests that modern birds appeared approximately 60–65 million years ago when the other lineages of dinosaurs (along with at least half of all terrestrial animals) were extinguished by a bolide impact. However, it is possible that modern birds were around much longer than that, although corroborating fossil evidence have yet to be found.

But scientists can also rely on another way to estimate the age of lineages: molecular clocks.

It was hypthosized that the DNA replication error-rate is consistent when averaged over time, across species and over various regions of the genome, similar to the regular ticking of a clock’s hands. Thus, molecular clocks rely on measuring these small changes in DNA sequences to determine the passage of time. It is generally accepted by the scientific community that DNA sequences diverge between distinct lineages by two percent every one million years.

There are several weaknesses associated with the fossil record; it isn’t complete and it only documents morphological changes long after species have diverged. But molecular clocks also have some inherent problems. For example, different lineages accumulate genetic changes at different rates, so estimates of evolutionary time will vary depending upon the lineages that are analyzed.

Nonetheless, even when these methodological weaknesses are addressed, the rocks-versus-clocks data still reveal a huge discrepancy between the estimated time of the dinosaur-modern bird split: the fossil record suggests it occurred in the Cenozoic (66 — 0 million years ago), while molecular clock data suggest it occurred deep within the Cretaceous (145.5 — 66 million years ago), or roughly 100 million years ago.

But several statistical methods have been developed that compensate for these different rates of molecular change across genomes, and a paper was just published that relies on these analytic methods for molecular clock data to estimate how long ago birds diverged from dinosaurs. The authors of this paper hoped that these new, more rigorous methods would generate data that narrow the discrepancies between the fossil record and the molecular data, thereby reducing the rock-clock conflict.

“What my colleagues and I did was apply all of these new methods to the problem of the origin of modern birds, with each method making different assumptions about how mutation rate changes across the [evolutionary] tree,” said University of Michigan graduate student Joseph Brown, who is the lead author on the paper.

Even though some discrepancies were expected because these two data sources rely on different stages of evolutionary change, the new analytic methods only strengthened the molecular clock conclusions that modern birds diverged from dinosaurs around 100 million years ago (Figure 1);

Figure 1: Different ways that fossil and molecular data date lineages. Time intervals defined by the horizontal dashed lines and vertical arrows pertain to age estimates for the divergence between hypothetical lineages X and Y. Even with a complete fossil record and perfect molecular clock a discrepancy is expected between fossil (FA) and molecular (MA) age estimates. As diagnostic morphological characters generally evolve (TMorphology) after species divergence (TSpecies), the fossil record will always underestimate (by δDiagnostic character) the true speciation time. Genetic data, on the other hand, will overestimate speciation time (by δCoalescence), as polymorphisms present during species divergence will coalesce some time in the past (TGene; related to the ancestral species effective population size). The genuine difference between molecular and morphological divergence times will thus be δTrue MA-FA. With a less complete fossil record, the oldest known fossil is unlikely to temporally correspond precisely to the origination of a diagnostic character delimiting X and Y, further decreasing FA by δOldest fossil. Under the more realistic scenario of lineage-specific rate heterogeneity and limited taxon/character sampling, errors associated with molecular methods (δClock error) may result in overestimation or underestimation of the true speciation time, although underestimates are bounded by the fossil constraint (δFossil error). The observed discrepancy in age estimates, δRealized MA-FA, may be considerably larger than expectations (δTrue MA-FA). [larger view]

“[W]e find strong support for an ancient origin of modern bird lineages, with many extant orders and families arising in the mid-Cretaceous, consistent with previous molecular estimates,” writes Brown and his colleagues.

Not only that, but this research addresses the conflict between which scientific methods are most accurate for studying the evolution of life.

“Rather than fighting across groups, we now have the joint goal of explaining this rock-clock gap,” Brown said. “Resolution of the issue will be fertile ground for future research for a while to come.”

These new findings provide more credence to some of the recent spectacular avian fossil discoveries, such as a fragmentary fossil from a parrot and from a loon (both of which were dismissed because they are thought to be inconclusive), and further, these data predict that there are yet more modern bird fossils waiting to be unearthed by palaeontologists.

Source:

Brown, J.W., Rest, J.S., García-Moreno, J., Sorenson, M.D., and Mindell, D.P. (2008). Strong mitochondrial DNA support for a Cretaceous origin of modern avian lineages, BMC Biology, 6(6) | doi:10.1186/1741–7007–6–6

Originally published at scienceblogs.com on 13 February 2008.