Reading a paper about the peopling of Sundaland by Homo erectus (Husson et al., 2022, Javanese Homo erectus on the move in SE Asia circa 1.8 Ma, I learned something about the time it took these ancient hominins to people this southeastern part of Asia, and it was much longer than I expected.
The authors report that " H. erectus reached Java and dwelled at Sangiran, Java, ca. 1.8 Ma.... The dispersal of H. erectus across Sundaland occurred over at least tens to hundreds kyr."
The map above shows how different the landscape was around 1.8 million years ago, with an emerged area spanning not only the Indonesian islands but also a land bridge from Vietnam into Sundaland, separating the seas of China and Thailand. It also depicts in orange and purple, possible entry routes from the mainland Asian area to Sanigran in Indonesia.
The time spent walking
The point they studied is how long did the trek of these erectus people take. After all, there are remains in China and also in Georgia, at the Dmanisi site, around 1.9 to 2 million years old, and the Indonesian remains are just 100,000 years younger. Let's see what the authors say (my comments are underlined) and I highlight some relevant parts:
" How long did the journey take? While least-cost paths are 6,000 km long on average from mainland Asia to Java (Fig. 4a), the mean travelled distances are 50 times longer for Markovian walks (327 x 103km), regardless of the north entry point, and almost always in excess of 100 x 103 km. The corresponding time span depends on the migration speed. Estimates are available for more recent palaeolithic hominins, ranging from 1 to 10 km/yr [I will comment on this point further down]. At similar speeds, the least-cost paths convert into an optimal travel time of 0.6 to 6 kyr, which is thus the minimal time to cross Sundaland. Markovian walks imply much longer durations before hominins reached Java, between 25-40 kyr at fast moving rates, and 250-400 kyr at slow moving rates (Fig. 4b). These relatively long delays at average or slow moving rates are difficult to reconcile with the early arrival to Java ca. 1.8 Ma, following departure from Georgia or China only a few 100 kyr before. We suggest that because these characteristic travel times scale with the characteristic time scale of changes of the physical environment, external forcings impelled hominins to migrate and boosted their displacement.... We find that H. erectus migrated across Sundaland at remarkably fast rates (>10 km/yr) with respect to their prior journey across Eurasia, and to the displacement rates of other lineages of the genus Homo"
In other words, they estimate that the joureny could take between 25,000 and 400,000 years, to move from northern Thailand or Vietnam to Sanigran in Indonesia. And say that the erectus people walked faster than that because there were external forces pushing them forward (?).
They adopt a quicker pace to fit the timeline of not more than 100,000 years between the oldest Asian specimens in China and Georgia, and those found in Indonesia. So, the authors suggest a slow pace across Asia from Georgia to East Asia, and then a very quick entry into Indonesia.
The paper assumes a speed of 10 km per year for Homo erectus which is the speed at which the fastest "recent palaeolithic hominins" move at (see further up, where it places that speed at 1 to 10 km/y). In my opion, this is quick.
So, just taking straight lines as a course (this would assume the travelers knew in advance where they were going, which they didn't), the distance however from northern Thailand to Java is roughly 3,000 km (2,000 miles). Which they. Taking figure 4b in the paper, at 10 km per year this journey would have required between 7 and 62 ky, on average 32 ky. If these people would have moved at the slower limit found in more recent palaeolithic hominins, they would have required 320 ky to do so (70 to 620 ky).
Interestingly, the modern humans of the Out Of Africa wave are said to have trekked from the Sinai (starting 60 kya) to Beringia in only 40,000 years. That is a much longer distance (10,700 km - 6,700 mi) than the one into Sunda from northern Thailand. So, moving at the pace of the erectus that peopled Sunda, to cover this distance that is 3 times longer, it would have taken almost 100 ky on average. So if they set out 60 kya, they still haven't arrived in America! And walking at the lowere speed limit, it would have taken 1 million years.
Something is wrong in the dispersal speeds.
The paper seems to understand this implication, because it ends with an interesting conclusion: "At a more general scale, our TCN ages obtained for the first appearance datum of H. erectus in Sangiran advocate for an early peopling of Sundaland by H. erectus, contemporary with their Chinese and Georgian counterparts, whose endocranial capacity was much smaller than specimens from Sangiran. These results reignite the long-standing controversy over the origin and dispersal pathways of archaic humans and invites a reexamination of the out-of-Africa paradigm, which provides a global roadmap for the dispersal of the genus Homo, but which one-way direction may be questioned."
Directionality suggests Asia-To-Africa migrations, and the smaller-brain sizes of contemporary erectus in China and Georgia vs. Indonesia requires some explanation. Why would the more advanced erectus be those most distant from Africa, in Sangiran?
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