Ardipithecus ramidus, human evolution, students and schools

Gary Hurd

LA Science Education Examiner

October 2, 2009

The last two days, and we can expect for the next two weeks, news reports have been filled with the spectacular Hominid fossils discovered in Ethiopia and published today in a special issue of Science Magazine.

The fossils represent the Hominin species, Ardipithecus ramidus and were nicknamed "Ardi." This will have much greater public penetration than many scientific discoveries because the AAAS has made their special edition public, and the announcement has been coordinated with the National Geographic Society, and news organizations such as CNN. This discovery has literally traveled the globe overnight. 

Your students will have questions for you that very few K-12 teachers are prepared to answer. I'll try to anticipate a few, and I'll reply to any questions posted below.

The first will be prompted by actually misleading, or at best poorly phrased headlines. These are from some surprising sources. The National Geographic News science editor, Jamie Shreeve, used the following headline for their article on Ardi, "Oldest "Human" Skeleton Found--Disproves "Missing Link."

There are several errors in a few words. These fossils are not "human." We limit "human" to the members of our Genus Homo, collectively called Hominidae often shortened to hominins.  The great apes, including humans are collectively called Hominids. "Ardi" is not a member of our genus, and is not a human. The notion of a "missing link" is falsely claimed by creationists to mean that there was a direct intermediate between modern humans and modern apes. This was never suggested by any evolutionary scientist, all the way back to Darwin. The discoverers of the newly reported fossils do think that even some of the common shared features of Chimps and Humans evolved independently. This is called convergent evolution, and is perhaps what Shreeve was refering to as "missing link" being falsified. This same error has appeared in other reports, such as by CNN.

So, the first likely question "Was this the first Human?" is answered, "No, it was one of the many early Hominids that were ancestors of Humans." The oldest member of our genus is still Homo habilus discovered in the Olduvai Gorge of Kenya and about 1.8 million years old.

These headlines also claim this is the "Oldest Human" with the emphasis on age. This is also incorrect. These fossils are exceptionally well dated and are 4.4 million years old. There are other known Hominid species older than Ardi. These are Ardipithecus kadabba, from Ethiopia about 5.5 million years old, Orrorin tugenensis from Kenya between 6.1 and 5.8 million years old, and Sahelanthropus tchadensis discovered in Chad and about 7 million years (Ma) old. This leads to the next most likely question you might hear from a student;

"How do they know how old it is?"

Ardipithecus ramadi fossils recently reported are all from a single three meter thick sedimentary deposit that is sandwiched between two volcanic ash, or tuff, layers.  The bottom layer is the Gàala("Camel") Tuff Complex (GATC) which has a ⁴⁰Ar/³⁹Ar age of4.419 ± 0.068 Ma. The capping strata is the Daam Aatu ("Baboon") BasalticTuff (DABT), which has a ⁴⁰Ar/³⁹Ar age of 4.416 ± 0.031Ma. The maximum age range known so far for these fossils is within 40,000 years. Argon dating is a radiometric method that measures the accumulation of argon 39 produced by the radioactive decay of argon 40 (see the excellent discription of ⁴⁰Ar/³⁹Ar dating provided by our reader, Readbeard in his comment below). Since the tuff was fully melted releasing its gasses, there is no question of residual argon to complicate the dates.

And the follow-up question is, "How do they know that radiometric dating works?"

We know this because electric lights work. We know this because any change in the universal physical constants great enough to affect dating results would necessarily have caused the earth to have recently melted, and penetrated with so much radiation that it would glow. We know this because radiometric dates agree with simple counting of annual events that accumulate in lakes, glaciers, and the ocean bottom. Radiometric dates also match with yearly events like tree rings.

Many students are influenced by creationism through their churches. Excellent resources for teachers on evolution are The National Center for Science Education, and Understanding Evolution

 

Ardipithecus ramidus Illustration courtesy J. H. Matternes via Science/AAAS